git-compat-util: drop now-unused `UNLEAK()` macro

git-compat-util: drop now-unused `UNLEAK()` macro

The `UNLEAK()` macro has been introduced with 0e5bba53af (add UNLEAK
annotation for reducing leak false positives, 2017-09-08) to help us
reduce the amount of reported memory leaks in cases we don't care about,
e.g. when exiting immediately afterwards. We have since removed all of
its users in favor of freeing the memory and thus don't need the macro
anymore.

Remove it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>