graft: use cmdstate.delete() to delete the state file
Previous patches start using state.cmdstate() class for statefiles. The class
has a function delete() to delete the state file. This patch replaces the
existing repo.vfs.unlink() with that delete() function.
Differential Revision: https://phab.mercurial-scm.org/D3656
(binary:application/octet-stream)