firmware: remove last vestiges of dabusb

dabusb was removed with commit dae86ccb ("[media] dabusb: remove
obsolete driver"), so remove the last vestiges of firmware and
documentation.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Cc: Rob Landley <rob@landley.net>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>