t1300: remove global config settings injected by test-lib.sh

t1300: remove global config settings injected by test-lib.sh

Since test-lib.sh now writes `safe.bareRepository=all` to the global
config when `WITH_BREAKING_CHANGES` is in effect, that entry shows
up in `git config --list` output. Tests in t1300 that expect exact
config contents then fail because of this unexpected extra line.

Unlike the working-tree contamination fixed in the preceding
commits, this is not about the file's existence but about its
content leaking into test expectations. Since t1300 does not use
bare repositories, simply remove the injected setting in a
preparatory step.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Assisted-by: Claude Opus 4.6
Signed-off-by: Junio C Hamano <gitster@pobox.com>