t1600-index: disable GIT_TEST_SPLIT_INDEX

t1600-index: disable GIT_TEST_SPLIT_INDEX

Tests in 't1600-index.sh' check that various bogus index version
values are recognized and an appropriate warning message is issued.
GIT_TEST_SPLIT_INDEX=1 is supposed to trigger index splitting
randomly, and thus might interfere [1] with these tests: splitting the
index means that two index files are written (the shared base index
and the split '.git/index'), and the same warning message is then
issued twice, failing these tests.

Unset GIT_TEST_SPLIT_INDEX in this test script to avoid such
interference.

[1] There is no such interference at the moment, because, alas,
    GIT_TEST_SPLIT_INDEX=1 is broken and never split the index.  There
    was no such interference in the past (before it broke) either,
    because the first index write with GIT_TEST_SPLIT_INDEX=1 never
    split the index, only the second write did.  A subsequent commit
    fixing GIT_TEST_SPLIT_INDEX will have all the details on this.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>