lib/ostruct: Remove unnecessary `__send__`

Patch by yuuji.yaginuma [Fix GH-1890]

Since `remove_method` is public.
Ref: https://bugs.ruby-lang.org/issues/14133

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64069 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/ostruct.rb

@@ -294,7 +294,7 @@ class OpenStruct
   def delete_field(name)
     sym = name.to_sym
     begin
-      singleton_class.__send__(:remove_method, sym, "#{sym}=")
+      singleton_class.remove_method(sym, "#{sym}=")
     rescue NameError
     end
     @table.delete(sym) do