Remove ORM use from DBSiteStore
This loses something of the genericity of the former DBSiteStore (i.e.
it's now tied to MediaWiki's database and sites table, and subclasses
and users can't easily override that), but nothing in core or extensions
in Gerrit was using that genericity so it's probably no big loss.
Further, T113034 (an RFC to actually use this 'site' stuff for its
original purpose) proposes getting rid of SiteStore anyway.