Makefile: Fix 'clean' target

Now that we have removed the DocBook files we need to not try and clean
that directory.

Reported-by: ericywl <midnight2903@gmail.com>
Reported-by: Jagan Teki <jagan@amarulasolutions.com>
Fixes: 78a88f79 ("doc: Replace DocBook with sphinx-based docs")
Signed-off-by: Tom Rini <trini@konsulko.com>