vm_method.c (release_method_definition): use rb_free_method_entry

We are not currently avoiding recursion by calling
release_method_definition, so use rb_free_method_entry directly
to make changing code easier in case we change the way
rb_method_entry_t is allocated.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47147 b2dd03c8-39d4-4d8f-98ff-823fe69b080e