revert --abort: do not leave behind useless sequencer-old directory

revert --abort: do not leave behind useless sequencer-old directory

The "git cherry-pick --abort" command currently renames the
.git/sequencer directory to .git/sequencer-old instead of removing it
on success due to an accident.  cherry-pick --abort is designed to
work in three steps:

 1) find which commit to roll back to
 2) call "git reset --merge <commit>" to move to that commit
 3) remove the .git/sequencer directory

But the careless author forgot step 3 entirely.  The only reason the
command worked anyway is that "git reset --merge <commit>" renames the
.git/sequencer directory as a secondary effect --- after moving to
<commit>, or so the logic goes, it is unlikely but possible that the
caller of git reset wants to continue the series of cherry-picks that
was in progress, so git renames the sequencer state to
.git/sequencer-old to be helpful while allowing the cherry-pick to be
resumed if the caller did not want to end the sequence after all.

By running "git cherry-pick --abort", the operator has clearly
indicated that she is not planning to continue cherry-picking.  Remove
the (renamed) .git/sequencer directory as intended all along.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>