gc: remove redundant check for gc_auto_threshold

gc: remove redundant check for gc_auto_threshold

Checking gc_auto_threshold in too_many_loose_objects() was added in
17815501a8 ("git-gc --auto: run "repack -A -d -l" as necessary.",
2007-09-17) when need_to_gc() itself was also reliant on
gc_auto_pack_limit before its early return:

    gc_auto_threshold <= 0 && gc_auto_pack_limit <= 0

When that check was simplified to just checking "gc_auto_threshold <=
0" in b14d255ba8 ("builtin-gc.c: allow disabling all auto-gc'ing by
assigning 0 to gc.auto", 2008-03-19) this unreachable code should have
been removed. We only call too_many_loose_objects() from within
need_to_gc() itself, which will return if this condition holds, and in
cmd_gc() which will return before ever getting to "auto_gc &&
too_many_loose_objects()" if "auto_gc && !need_to_gc()" is true
earlier in the function.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>