Remove the alias parameter in UnresolvedRelation

UnresolvedRelation has a second argument named "alias", for assigning the relation an alias. I think we can actually remove it and replace its use with a SubqueryAlias.

This would actually simplify some analyzer code to only match on SubqueryAlias. For example, the broadcast hint pull request can have one fewer case https://github.com/apache/spark/pull/16925/files.