[iOS] Add sign out without action sheet for UNO users in manage accounts

For signed in not syncing accounts, signing out from manage accounts UI
should not offer to keep/clear data, just signs out directly.

This CL updates the logic of signing out in this case by:
- Unifying the user action name "Signin_Signout".
- Removing unused method logSignoutMetricsWithForceClearData.
- Introducing signOut in accountsTVC.
- Showing a snackbar when signing out from the unified settings UI.
Note: showing this snackbar when signing out from manage accounts UI is
TBD.

Snackbar screenshot in https://crbug.com/1447017#c18

Bug: 1447017
Change-Id: I63b5be6a70b9715b273234e9134651c4491e62b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4624572
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Jood Hajeer <jood@google.com>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1160676}