[Sync] Get rid of KeyDerivationMethod::UNSUPPORTED

NigoriSyncBridge always rejects remote NigoriSpecifics with unknown key
derivation method (corresponds to KeyDerivationMethod::UNSUPPORTED) and
goes into ModelError state, thus enum uses beyond handling/validation
of the remote NigoriSpecifics can't encounter UNSUPPORTED.

This CL removes this enum value and adapts optional where appropriate.

Fixed: 1379853
Change-Id: I03a9d5ce34a5493cfc18f696ffa347a0bd8854d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4000104
Auto-Submit: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1067333}