blockjob: Fix assertion in block_job_finalize()

Every job gets a non-NULL job->txn on creation, but it doesn't
necessarily keep it until it is decommissioned: Finalising a job removes
it from its transaction. Therefore, calling 'blockdev-job-finalize' a
second time on an already concluded job causes an assertion failure.

Remove job->txn from the assertion in block_job_finalize() to fix this.
block_job_do_finalize() still has the same assertion, but if a job is
already removed from its transaction, block_job_apply_verb() will
already error out before we run into that assertion.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>