Remove unused Block#end_tag method.

Although the method is called, it is defined with an empty body and not
overridden to do anything else.

lib/liquid/block.rb

@@ -23,10 +23,7 @@ module Liquid
 
                 # if we found the proper block delimiter just end parsing here and let the outer block
                 # proceed
-                if block_delimiter == $1
-                  end_tag
-                  return
-                end
+                return if block_delimiter == $1
 
                 # fetch the tag from registered blocks
                 if tag = Template.tags[$1]
@@ -77,9 +74,6 @@ module Liquid
       all_warnings
     end
 
-    def end_tag
-    end
-
     def unknown_tag(tag, params, tokens)
       case tag
       when 'else'.freeze