docs: Avoid warning on cleandocs

Recent Makefile changes added an rm command without the requisite "-f",
leading to warnings if the files do not exist.  Make it be quiet again.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Jonathan Corbet 2016-11-16 15:38:03 -07:00
commit c54b6b3779