[ruby/readline] Suppress constant redefinition warning

When already set by `use_lib_reline` in test/readline/helper.rb of
readline-ext.

https://github.com/ruby/readline/commit/0e3ca3b217

lib/readline.rb

@@ -2,5 +2,6 @@ begin
   require 'readline.so'
 rescue LoadError
   require 'reline' unless defined? Reline
+  Object.send(:remove_const, :Readline) if Object.const_defined?(:Readline)
   Readline = Reline
 end