Similarly to previous patch, let's cleanup gitlab-backup temporary
folder always unconditionally in the presence of errors. Keeping $tmpd
on error was not preventing further gitlab-backup run to proceed, but it
can quickly eat up disk space if there are many such runs. If debugging
is needed one can comment the cleanup, but by default let's be
production friendly out of the box.

Based on patch by @alain.takoudjou:
!4

Original description from Alain:

---- 8< ----
When script exit, remove tmp backup folder which are not longuer needed.
Keep this folder when backup is failing will contribute to fill the disk
of server. backup.locked is also removed, because we want to
automatically retry gitlab-backup if previous backup failed, without
human action. If the file is not removed automatically, backup is
blocked until someone remove it.