vm_method.c: fix method_removed

* vm_method.c: call method_removed hook on called class, not on
  prepending iclass.  [ruby-core:52207] [Bug #7843]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39235 b2dd03c8-39d4-4d8f-98ff-823fe69b080e