Makefile: drop GEN_HDRS

Makefile: drop GEN_HDRS

When ebb7baf0 ("Makefile: add a hdr-check target", 2018-09-19)
implemented hdr-check target, it wanted to leave some header files
exempt from the stricter check the target implements, and added
GEN_HDRS macro.

This however is probably a bad move for two reasons:

 - If we value the header cleanliness check, we eventually want to
   teach our header generating scripts to produce clean headers.
   Keeping the blanket "generated headers can be left as dirty as we
   want" exception does not nudge us in the right direction.

 - There is a list of generated header files, GENERATED_H, which is
   used to keep track of dependencies.  Presence of GEN_HDRS that is
   too similarly named would confuse developers who are adding new
   generated header files which list to add theirs.

 - Even though unicode-width.h could be generated using a contrib/
   script, as far as our build infrastructure is concerned, it is a
   source file that is tracked in the source control system.  Its
   presence in GEN_HDRS list is doubly misleading.

Get rid of GEN_HDRS, which is used only once to list the headers we
do not run hdr-check test on, and instead explicitly list that the
ones, either tracked or generated, that we exempt from the test.

This allows GENERATED_H to be the sole "here are build artifact
header files that are expendable" list, so use it in the clean
target to $(RM) them.

Signed-off-by: Junio C Hamano <gitster@pobox.com>