sequencer: unlink autostash in apply_autostash()

Explicitly remove autostash file in apply_autostash() once it has been
applied successfully.

This is currently a no-op because the only users of this function will unlink
the state (including the autostash file) after this function runs.
However, in the future, we will introduce a user of the function that
does not explicitly remove the state so we do it here.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -3748,6 +3748,7 @@ int apply_autostash(const char *path)
 				  " \"git stash drop\" at any time.\n"));
 	}
 
+	unlink(path);
 	strbuf_release(&stash_oid);
 	return ret;
 }