gc.c (rb_free_m_tbl): mark function as static

* gc.c (rb_free_m_tbl): mark function as static
* method.h (rb_free_m_tbl): remove prototype

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46055 b2dd03c8-39d4-4d8f-98ff-823fe69b080e