t/perf: drop p5311's lookup-table permutation

p5311 measures the cost of serving a fetch from a bitmapped pack and indexing the resulting pack on the client. Since 761416ef91d (bitmap-lookup-table: add performance tests for lookup table, 2022-08-14), p5311 effectively runs itself twice: once with the bitmap's lookup table extension enabled, and again with it disabled. This comparison has served its useful purpose, as the lookup table is almost four years old, and the de-facto default in server-side Git deployments. A following commit will want to test a different combination (repacking with and without '--path-walk' instead of the lookup table). Instead of multiplying the current test count by two again to produce four variations of `test_fetch_bitmaps()`, drop the lookup table option to reduce the number of perf tests we run. Retain `test_fetch_bitmaps()` itself, since we will use this in the future for the new parameterization. (As an aside, a future commit outside of this series will adjust the default value of 'pack.writeBitmapLookupTable' to "true", matching the de-facto norm for deployments where the existence of bitmap lookup tables is meaningful. Punt on that to a later series and instead make the minimal change for now.) Suggested-by: Derrick Stolee <stolee@gmail.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>