t: prepare `git config --unset` calls for `set -e`

t: prepare `git config --unset` calls for `set -e`

We have a couple of calls to `git config --unset` that ultimately end up
as no-ops as the configuration variables aren't set (anymore) in the
first place. These calls are mostly intended to recover unconditionally
from tests that may have executed only partially, but they'll ultimately
fail during a normal test run.

This hasn't been a problem until now as we aren't running tests with
`set -e`. This is about to change though, so let's silence the case
where we cannot unset the config keys.

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