Makefile: remove an out-of-date comment

Makefile: remove an out-of-date comment

This comment added in dfea575017 (Makefile: lazily compute header
dependencies, 2010-01-26) has been out of date since
92b88eba9f (Makefile: use `git ls-files` to list header files, if
possible, 2019-03-04), when we did exactly what it tells us not to do
and added $(GENERATED_H) to $(OBJECTS) dependencies.

The rest of it was also somewhere between inaccurate and outdated,
since as of b8ba629264 (Makefile: fold MISC_H into LIB_H, 2012-06-20)
it's not followed by a list of header files, that got moved earlier in
the file into LIB_H in 60d24dd255 (Makefile: fold XDIFF_H and VCSSVN_H
into LIB_H, 2012-07-06).

Let's just remove it entirely, to the extent that we have anything
useful to say here the comment on the
"USE_COMPUTED_HEADER_DEPENDENCIES" variable a few lines above this
change does the job for us.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>