[sync] Remove UserActionableError::kGenericUnrecoverableError

Unrecoverable errors are unactionable and hence are not displayed to the
user. It does not make sense to include this in UserActionableError.
Callers should instead use GetDisableReasons() to enquire if sync is
disabled due to an unrecoverable error.

Note: There exist some flows which lead to showing UI elements for
kGenericUnrecoverableError but these flows are now unreachable on iOS
because of transition from sync to sign-in.

Bug: 349586095
Change-Id: Ief5232f813d983a908343298eed8671b7c9db258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5659596
Commit-Queue: Ankush Singh <ankushkush@google.com>
Reviewed-by: David Roger <droger@chromium.org>
Auto-Submit: Ankush Singh <ankushkush@google.com>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1322134}