Makefile: drop CHECK_HEADER_DEPENDENCIES code

This code was useful when we kept a static list of header
files, and it was easy to forget to update it. Since the last
commit, we generate the list dynamically.

Technically this could still be used to find a dependency
that our dynamic check misses (e.g., a header file without a
".h" extension).  But that is reasonably unlikely to be
added, and even less likely to be noticed by this tool
(because it has to be run manually)., It is not worth
carrying around the cruft in the Makefile.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>