Remove Debug Statement for Win/MinGW (#5283)

ruby/ext/google/protobuf_c/extconf.rb

@@ -2,9 +2,7 @@
 
 require 'mkmf'
 
-unless RUBY_PLATFORM =~ /mswin|mingw/
-  $CFLAGS += " -std=c99 -O3 -DNDEBUG"
-end
+$CFLAGS += " -std=c99 -O3 -DNDEBUG"
 
 
 if RUBY_PLATFORM =~ /linux/