DocBook: Reduce noise from make cleandocs

I've got a harmless warning when running make cleandocs on an already
cleaned tree:
  Documentation/DocBook/media/Makefile:28: recipe for target 'cleanmediadocs' failed
  make[1]: [cleanmediadocs] Error 1 (ignored)

Suppress this by passing -f to rm.

Acked-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>