Stop trying to remove every file on extraction

When we're extracting a gem, it should be extracted to an empty
directory.  Trying to remove every file before extracting the file
greatly slows the tar extraction process.

This change increases tar extraction speed by about 10%:

Master branch:

```
aaron@tc ~/g/tartest (main)> ruby -I../rubygems/lib:../vernier/lib test.rb
{MEAN: 0.6701955500058829}
{stddev: 0.029875687861609046}
{"mib/s": 20.88942548167786}
```

This commit:

```
aaron@tc ~/g/tartest (main)> ruby -I../rubygems/lib:../vernier/lib test.rb
{MEAN: 0.6002237499691546}
{stddev: 0.03722422666707127}
{"mib/s": 23.324635189326408}
```

Author: tenderlove

lib/rubygems/package.rb

@@ -436,8 +436,6 @@ def extract_tar_gz(io, destination_dir, pattern = "*") # :nodoc:
           symlinks << [full_name, link_target, destination, real_destination]
         end
 
-        FileUtils.rm_rf destination
-
         mkdir =
           if entry.directory?
             destination