Makefile: clean *.o files we create

Makefile: clean *.o files we create

The part that removes object files in the 'clean' target predates
various Makefile macros that list object files we create, and
instead removes the objects with shell glob, perpetually requiring
updates whenever a new location that builds object files is added.

Simplify the target by removing $(OBJECTS), which is supposed to
have all the objects we create during the build.

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