Turn off `enable_debug` flag in `build_config.rb` by default.

This will improve the default performance of mruby; fix #4045

build_config.rb

@@ -8,7 +8,8 @@ MRuby::Build.new do |conf|
     toolchain :gcc
   end
 
-  enable_debug
+  # Turn on `enable_debug` for better debugging
+  # enable_debug
 
   # Use mrbgems
   # conf.gem 'examples/mrbgems/ruby_extension_example'