Remove counterproductive optimization

I think it's debatable which is the most common usage of
`FileUtils.mkdir_p`, but even assuming the most common use case is
creating a folder when it doesn't previously exist but the parent does,
this optimization doesn't seem to have a noticiable effect there while
harming other use cases.

For benchmarks, I created this script

```ruby
require "benchmark/ips"

Benchmark.ips do |x|
  x.report("old mkdir_p - exists") do
    FileUtils.mkdir_p "/tmp"
  end

  x.report("new_mkdir_p - exists") do
    FileUtils.mkdir_p_new "/tmp"
  end

  x.compare!
end

FileUtils.rm_rf "/tmp/foo"

Benchmark.ips do |x|
  x.report("old mkdir_p - doesnt exist, parent exists") do
    FileUtils.mkdir_p "/tmp/foo"
    FileUtils.rm_rf "/tmp/foo"
  end

  x.report("new_mkdir_p - doesnt exist, parent exists") do
    FileUtils.mkdir_p_new "/tmp/foo"
    FileUtils.rm_rf "/tmp/foo"
  end

  x.compare!
end

Benchmark.ips do |x|
  x.report("old mkdir_p - doesnt exist, parent either") do
    FileUtils.mkdir_p "/tmp/foo/bar"
    FileUtils.rm_rf "/tmp/foo"
  end

  x.report("new_mkdir_p - doesnt exist, parent either") do
    FileUtils.mkdir_p_new "/tmp/foo/bar"
    FileUtils.rm_rf "/tmp/foo"
  end

  x.compare!
end

Benchmark.ips do |x|
  x.report("old mkdir_p - more levels") do
    FileUtils.mkdir_p "/tmp/foo/bar/baz"
    FileUtils.rm_rf "/tmp/foo"
  end

  x.report("new_mkdir_p - more levels") do
    FileUtils.mkdir_p_new "/tmp/foo/bar/baz"
    FileUtils.rm_rf "/tmp/foo"
  end

  x.compare!
end
```

and copied the method with the "optimization" removed as
`FileUtils.mkdir_p_new`. The results are as below:

```
Warming up --------------------------------------
old mkdir_p - exists    15.914k i/100ms
new_mkdir_p - exists    46.512k i/100ms
Calculating -------------------------------------
old mkdir_p - exists    161.461k (± 3.2%) i/s -    811.614k in   5.032315s
new_mkdir_p - exists    468.192k (± 2.9%) i/s -      2.372M in   5.071225s

Comparison:
new_mkdir_p - exists:   468192.1 i/s
old mkdir_p - exists:   161461.0 i/s - 2.90x  (± 0.00) slower

Warming up --------------------------------------
old mkdir_p - doesnt exist, parent exists
                         2.142k i/100ms
new_mkdir_p - doesnt exist, parent exists
                         1.961k i/100ms
Calculating -------------------------------------
old mkdir_p - doesnt exist, parent exists
                         21.242k (± 6.7%) i/s -    107.100k in   5.069206s
new_mkdir_p - doesnt exist, parent exists
                         19.682k (± 4.2%) i/s -    100.011k in   5.091961s

Comparison:
old mkdir_p - doesnt exist, parent exists:    21241.7 i/s
new_mkdir_p - doesnt exist, parent exists:    19681.7 i/s - same-ish: difference falls within error

Warming up --------------------------------------
old mkdir_p - doesnt exist, parent either
                       945.000  i/100ms
new_mkdir_p - doesnt exist, parent either
                         1.002k i/100ms
Calculating -------------------------------------
old mkdir_p - doesnt exist, parent either
                          9.689k (± 4.4%) i/s -     49.140k in   5.084342s
new_mkdir_p - doesnt exist, parent either
                         10.806k (± 4.6%) i/s -     54.108k in   5.020714s

Comparison:
new_mkdir_p - doesnt exist, parent either:    10806.3 i/s
old mkdir_p - doesnt exist, parent either:     9689.3 i/s - 1.12x  (± 0.00) slower

Warming up --------------------------------------
old mkdir_p - more levels
                       702.000  i/100ms
new_mkdir_p - more levels
                       775.000  i/100ms
Calculating -------------------------------------
old mkdir_p - more levels
                          7.046k (± 3.5%) i/s -     35.802k in   5.087548s
new_mkdir_p - more levels
                          7.685k (± 5.5%) i/s -     38.750k in   5.061351s

Comparison:
new_mkdir_p - more levels:     7685.1 i/s
old mkdir_p - more levels:     7046.4 i/s - same-ish: difference falls within error
```

I think it's better to keep the code simpler is the optimization is not
so clear like in this case.

lib/fileutils.rb

@@ -211,14 +211,6 @@ def mkdir_p(list, mode: nil, noop: nil, verbose: nil)
     list.each do |item|
       path = remove_trailing_slash(item)
 
-      # optimize for the most common case
-      begin
-        fu_mkdir path, mode
-        next
-      rescue SystemCallError
-        next if File.directory?(path)
-      end
-
       stack = []
       until File.directory?(path)
         stack.push path