Add field_write_barrier when an method is defined.
Bug fix.
removed unused mrb_funcargv_t
removed unused mrb_check_funcall
Merge pull request #806 from cremno/unused-stuff

Unused func prototype and typedef
Merge pull request #805 from masamitsu-murase/add_field_write_barrier_to_guard_method_from_gc

Add field_write_barrier when an method is defined.