Don't stub synchronization on MRI

Having a GIL does not mean you don't have to perform any synchronization
at all. While most Ruby code won't be able to run in parallel native
extensions _can_ run in parallel by calling functions such as
rb_thread_check_ints() (which basically pauses the current thread when
needed).

Depending on the Ruby environment to figure out if you need
synchronization or not is _extremely_ dangerous. While this can work
most of the time, once in a while you'll get weird NoMethodErrors or
worse: segmentation faults, depending on the database adapter being
used. In particular the PostgreSQL adapter (which uses the "pg" Gem)
suffers from this as connection objects created by the "pg" Gem (or
really any other object it provedes) are not thread-safe.

While synchronizing access on MRI will have a slight performance impact,
it's either that or errors/segmentation faults.

lib/sequel/core.rb

@@ -293,24 +293,14 @@ def self.string_to_time(string)
     end
   end
 
-  if defined?(RUBY_ENGINE) && RUBY_ENGINE != 'ruby'
-  # :nocov:
-    # Mutex used to protect mutable data structures
-    @data_mutex = Mutex.new
-
-    # Unless in single threaded mode, protects access to any mutable
-    # global data structure in Sequel.
-    # Uses a non-reentrant mutex, so calling code should be careful.
-    def self.synchronize(&block)
-      @single_threaded ? yield : @data_mutex.synchronize(&block)
-    end
-  # :nocov:
-  else
-    # Yield directly to the block.  You don't need to synchronize
-    # access on MRI because the GVL makes certain methods atomic.
-    def self.synchronize
-      yield
-    end
+  # Mutex used to protect mutable data structures
+  @data_mutex = Mutex.new
+
+  # Unless in single threaded mode, protects access to any mutable
+  # global data structure in Sequel.
+  # Uses a non-reentrant mutex, so calling code should be careful.
+  def self.synchronize(&block)
+    @single_threaded ? yield : @data_mutex.synchronize(&block)
   end
 
   # Uses a transaction on all given databases with the given options. This: