Don't log warning if codehost is unsupported in ChangesetSyncer/RateLimiter

Created by: mrnugget

(cc @ryanslade)

This fixes https://github.com/sourcegraph/sourcegraph/issues/9779 by not not logging a warning when a codehost is either

a) not supported as a ChangesetSyncer b) not supported with a custom rate limiter

In addition to that I also changed the SyncRegistry according to this comment (https://github.com/sourcegraph/sourcegraph/pull/9544/files#r402890686) and not create syncers for unsupported code hosts.