registration: Fix 'ResourceLoaderWikiModule' schema validation

registration: Fix 'ResourceLoaderWikiModule' schema validation

There is a full schema for ResourceModules when the
class: ResourceLoaderWikiModule is used. Except it doesn't actually work,
since it it fails validating against that schema, it will continue to check
the other anyOf schemas, find the arbitrary class one, and pass that one.

Blacklisting ResourceLoaderWikiModule from the arbitrary class schema means
that it has to pass the specified schema for it, or fail validation.

I verified that all extensions and skins in Gerrit pass validation with this
patch.

Change-Id: Icb862088c0ca1f89ed5a57a4286dd9049d213021