selftests/bpf: Clean up fmod_ret in bench_rename test script

Running the bench_rename test script, the following error occurs:

  # ./benchs/run_bench_rename.sh
  base      :    0.819 ± 0.012M/s
  kprobe    :    0.538 ± 0.009M/s
  kretprobe :    0.503 ± 0.004M/s
  rawtp     :    0.779 ± 0.020M/s
  fentry    :    0.726 ± 0.007M/s
  fexit     :    0.691 ± 0.007M/s
  benchmark 'rename-fmodret' not found

The bench_rename_fmodret has been removed in commit b000def2
("selftests: Remove fmod_ret from test_overhead"), thus remove it
from the runners in the test script.

Fixes: b000def2 ("selftests: Remove fmod_ret from test_overhead")
Signed-off-by: Yipeng Zou <zouyipeng@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230814030727.3010390-1-zouyipeng@huawei.com