Cleanup obsolete doc build targets

- the *-check-docs target doesn't do anything the other targets don't
- online-* are, I think, the old d.a.c targets but the ds-* targets are
  what's currently used
- the hidden-* target isn't meaningfully different from others, either

Test: presubmit
Change-Id: Icab977d40917aa14c629629965c6126e20f4f3ee