t1091: disable split index

t1091: disable split index

In 61feddcdf28 (tests: disable GIT_TEST_SPLIT_INDEX for sparse index
tests, 2021-08-26), it was already called out that the split index
feature is incompatible with the sparse index feature, and its commit
message wondered aloud whether more checks would be required to ensure
that the split index and sparse index features aren't enabled at the
same time.

We are about to introduce such additional checks, and indeed, t1091
would utterly fail with them. Therefore, let's preemptively disable the
split index for the entirety of t1091.

This partially reverts above-mentioned patch because it covered only one
test case whereas we want to cover the entire test script.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>