Remove needless library lock with gem methods.

  This code inserted at 978cd90
  But current rdoc/text.rb implementation did not use JSON library.

Author: hsbt

lib/rdoc/text.rb

@@ -5,16 +5,6 @@
 
 require 'strscan'
 
-##
-# For RDoc::Text#snippet
-
-begin
-  gem 'json'
-rescue NameError => e # --disable-gems
-  raise unless e.name == :gem
-rescue Gem::LoadError
-end
-
 ##
 # Methods for manipulating comment text