backup: don't acquire aio_context in backup_clean

backup: don't acquire aio_context in backup_clean

All code-paths leading to backup_clean (via job_clean) have the job's
context already acquired. The job's context is guaranteed to be the same
as the one used by backup_top via backup_job_create.

Since the previous logic effectively acquired the lock twice, this
broke cleanup of backups for disks using IO threads, since the BDRV_POLL_WHILE
in bdrv_backup_top_drop -> bdrv_do_drained_begin would only release the lock
once, thus deadlocking with the IO thread.

This is a partial revert of 0abf2581717a19.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200407115651.69472-4-s.reiter@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>