p5312: removed duplicate performance test script

When the reachability bitmap format learned to read and write a lookup
table containing the set of commits which received reachability bitmaps,
commit 761416e (bitmap-lookup-table: add performance tests for lookup
table, 2022-08-14) added that mirrored p5310 but with reverse indexes
enabled.

Later on in a8dd7e0 (config: enable `pack.writeReverseIndex` by
default, 2023-04-12), we enabled reverse indexes by default, which made
these two tests indistinguishable from one another. Commit a8dd7e0
should have removed p5312 as a duplicate, but didn't do so.

Correct that by removing p5312 as a functional duplicate of p5310.

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

Author: ttaylorr