gconf-helpers: remove commented-out code (#if UNUSED)

These code paths are now disabled for 6 years since
2a4aed586c87b7846a5b890955b97319bebf2674 . Just remove them.

Signed-off-by: Thomas Haller <thaller@redhat.com>
build: update m4/compiler_warnings.m4

Update m4 macro from current NetworkManager master branch;
with one difference: keep also -Wstrict-prototypes (which
NM does not enable).

Signed-off-by: Thomas Haller <thaller@redhat.com>