block: mirror - remove code cruft that has no function

block: mirror - remove code cruft that has no function

Originally, this built up the error message with the backing filename,
so that errp was set as follows:
    error_set(errp, QERR_OPEN_FILE_FAILED, backing_filename);

However, we now propagate the local_error from the
bdrv_open_backing_file() call instead, making these 2 lines useless
code.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>