Bug 31740: Remove some unnecessary RemoteSettings instances

More concretely, SearchService.jsm 'hijack-blocklists' and
url-classifier-skip-urls.

Avoid creating instance for 'anti-tracking-url-decoration'.

If prefs are disabling their usage, avoid creating instances for
'cert-revocations' and 'intermediates'.

Do not ship JSON dumps for collections we do not expect to need. For
the ones in the 'main' bucket, this prevents them from being synced
unnecessarily (the code in remote-settings does so for collections
in the main bucket for which a dump or local data exists). For the
collections in the other buckets, we just save some size by not
shipping their dumps.

We also clear the collections database on the v2 -> v3 migration.