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

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>