USB: legousbtower: remove tower_abort_transfers()

Drop the tower_abort_transfers() function which is now only called from
release and instead explicitly kill the two URBs.

This incidentally also fixes the outdated comment about freeing memory.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191105084152.16322-9-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>