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

This argument was added in 7cae762 (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

t/perf/p2000-sparse-operations.sh

@@ -124,6 +124,6 @@ test_perf_on_all git read-tree -mu HEAD
 test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove $SPARSE_CONE/a
 test_perf_on_all "git rm -f $SPARSE_CONE/a && git checkout HEAD -- $SPARSE_CONE/a"
-test_perf_on_all git grep --cached --sparse bogus -- "f2/f1/f1/*"
+test_perf_on_all git grep --cached bogus -- "f2/f1/f1/*"
 
 test_done