gc: remove `--all-commits` options from usage output

[jorgeorpinel]

Per iterative/dvc.org#730 (comment)

$ dvc gc --help
usage: ...

optional arguments:
  -h, --help            show this help message and exit
  -q, --quiet           Be quiet.
  -v, --verbose         Be verbose.
  -a, --all-branches    Keep data files for the tips of all git branches.
  -T, --all-tags        Keep data files for all git tags.
  --all-commits         Keep data files for all commits.     <-- HIDE FOR NOW.
  -c, --cloud           Collect garbage in remote repository.
  ...

Related: #2325

[efiop]

@Suor Please check your mentions iterative/dvc.org#730 (comment) . This flag should be hidden for now, not removed.