Merge pull request #2173 from suzukaze/delete-unuse-variable

Delete unused variable in Array#delete_if

mrbgems/mruby-array-ext/mrblib/array.rb

@@ -617,7 +617,6 @@ class Array
     return to_enum :delete_if unless block_given?
 
     idx = 0
-    len = self.size
     while idx < self.size do
       if block.call(self[idx])
         self.delete_at(idx)