AccountState grew a per-account, generic cache of properties. This is bad, because it makes AccountState mutable (values are added). AccountState is part of AccountCacheImpl's cache, so it should be immutable. We removed all callers on googlesource.com and there are no callers in Gerrit core or core plugins, so this commit removes this functionality. Change-Id: Icc0cac05466d650e89626d2493d999ff089d5ea9