p2000: remove stray '--sparse' flag from test

This argument was added in 7cae7627c45 (builtin/grep.c: integrate with sparse index, 2022-09-22), but it was a carry-over from an earlier version where the --sparse flag was added to the 'git grep' builtin. This argument does not exist, so currently the p2000-sparse-operations.sh performance test script fails when reaching this step. With this fix, the script works with these numbers for my copy of the Git source code repository: Test HEAD ------------------------------------------------------------ 2000.30: git grep --cached ... (full-v3) 0.34(1.20+0.14) 2000.31: git grep --cached ... (full-v4) 0.31(1.15+0.13) 2000.32: git grep --cached ... (sparse-v3) 0.26(1.13+0.12) 2000.33: git grep --cached ... (sparse-v4) 0.27(1.13+0.12) Signed-off-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>