Remove documentation about requiring libraries in rdoc/liquid templates

It is better to require tilt/{rdoc,liquid} than the underlying
libraries (especially in the rdoc case), as mentioned in the README.

lib/tilt/liquid.rb

@@ -13,9 +13,6 @@ module Tilt
   # to #to_h it will be ignored.
   #
   # LiquidTemplate does not support yield blocks.
-  #
-  # It's suggested that your program require 'liquid' at load
-  # time when using this template engine.
   class LiquidTemplate < Template
     def prepare
       @options[:line_numbers] = true unless @options.has_key?(:line_numbers)

lib/tilt/rdoc.rb

@@ -6,14 +6,6 @@
 require 'rdoc/options'
 
 # RDoc template. See: https://github.com/ruby/rdoc
-#
-# It's suggested that your program run the following at load time when
-# using this templae engine in a threaded environment:
-#
-#   require 'rdoc'
-#   require 'rdoc/markup'
-#   require 'rdoc/markup/to_html'
-#   require 'rdoc/options'
 Tilt::RDocTemplate = Tilt::StaticTemplate.subclass do
   RDoc::Markup::ToHtml.new(RDoc::Options.new, nil).convert(@data).to_s
 end