Makefile: remove redundant GIT-CFLAGS dependency from "sparse"

Makefile: remove redundant GIT-CFLAGS dependency from "sparse"

The "sparse" target needed the GIT-CFLAGS dependency before my
c234e8a0ecf (Makefile: make the "sparse" target non-.PHONY,
2021-09-23), but since then it depends on the corresponding *.o files,
which in turn depend on the correct header files, as well as on
GIT-CFLAGS. There's no need to re-state this dependency here.

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