kernel-doc: drop the -filelist option, it doesn't work

I also found the -filelist option, but apparently the implementation
is broken, and it was broken from the very first git commit.
For the -filelist option I suggest the removal (I wasn't able to find
any users of it, moreover it's not even listed in the
usage() output, so presumably nobody knows about it).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>