Makefile: don't try to construct CLEAN_FILES based on config

All of the clean targets (clean, dist-clean, mrproper) are run without
loading a configuration. That means that any conditional modification of
CLEAN_FILES will either always apply or never apply and so shouldn't be
in a conditional in the first place.

Since CLEAN_FILES is allowed to list nonexistent files, just add
everything to it unconditionally to fix the issue.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>