sequencer: unlink autostash in apply_autostash()

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>