Turn of `enable_debug` by default; ref #5096

It was once turned off by #4045

target/host.rb

@@ -80,7 +80,7 @@ MRuby::Build.new do |conf|
   # conf.file_separator = '/'
 
   # Turn on `enable_debug` for better debugging
-  conf.enable_debug
+  # conf.enable_debug
   conf.enable_bintest
   conf.enable_test
 end