search: reduce queries in single tag searches (#4120).

Avoid some queries used in wiki page excerpts:

* Only try to load the artist if the tag is an artist tag.
* Avoid using `exists?` queries for wiki pages.
* Bugfix: don't show wiki excerpts for deleted wikis.

app/logical/post_sets/base.rb

@@ -1,9 +1,5 @@
 module PostSets
   class Base
-    def has_wiki?
-      false
-    end
-
     def raw
       false
     end
@@ -12,10 +8,6 @@ module PostSets
       nil
     end
 
-    def has_artist?
-      false
-    end
-
     def artist
       nil
     end