recipe: drop useless keep of firmware files

Since commit f838a724, we were both keeping *-firmware-* files in
/usr/share/docs and not installing them with --excludedocs.  This
patch removes the conflicting keep.

Change-Id: I9830d0b6ec0cdae2c711a95aeb0ad996e30de447
Signed-off-by: Mike Burns <mburns@redhat.com>