Remove kChromadAvailable feature flag

Clean up the kChromadAvailable feature flag, which was used to disable
Chromad in M110 (b/259180126). While removing the flag, we keep the
behaviour for the disabled value.

As a result, devices that are already in AD mode will remain disabled,
displaying a custom error message to the user. Without the flag, there
is no way to re-enable Chromad login in the new Chrome builds.
Therefore, this CL also removes login tests that are specific to Active
Directory management mode.

Bug: b:263367348
Change-Id: I1eae068a1ac3e4902af2788b36c4e1d0a22cb548
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4427850


Reviewed-by: Roman Sorokin <rsorokin@google.com>
Commit-Queue: Felipe Andrade <fsandrade@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1130493}