remove empty rb_gc_mark_parser function

* parse.y (rb_gc_mark_parser): remove, empty since r8758
* internal.h: ditto, not usable from extensions since 2.0.0
* gc.c (gc_mark_roots): remove checkpoint for parser

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46239 b2dd03c8-39d4-4d8f-98ff-823fe69b080e