docs: Makefile: remove no-ops targets

After removal of DocBook, those targets are bogus.

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
3 files changed