mirror: drop local_err in mirror_complete

There is errp passed in, so no need for local_err and error_propagate.
Also drop the backing_filename which is set but unused since 34b5d2c.

Signed-off-by: Fam Zheng <famz@redhat.com>

--
v2: fix typo in subject line.
    drop backing_filename.
    (Thanks Eric)