p5326: don't set core.multiPackIndex unnecessarily

When this performance test was originally written, `core.multiPackIndex`
was not the default and thus had to be enabled. But now that we have
18e449f (midx: enable core.multiPackIndex by default, 2020-09-25), we
no longer need this.

Drop the unnecessary setup (even though it's not hurting anything, it is
unnecessary at best and confusing at worst).

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

t/perf/p5326-multi-pack-bitmaps.sh

@@ -6,10 +6,6 @@ test_description='Tests performance using midx bitmaps'
 
 test_perf_large_repo
 
-test_expect_success 'enable multi-pack index' '
-	git config core.multiPackIndex true
-'
-
 # we need to create the tag up front such that it is covered by the repack and
 # thus by generated bitmaps.
 test_expect_success 'create tags' '