autocomplete: fixup uses of tag_aliases.post_count.

Fixup for 64728c89d.

app/logical/tag_autocomplete.rb

@@ -105,8 +105,8 @@ module TagAutocomplete
       .where("tag_aliases.antecedent_name LIKE ? ESCAPE E'\\\\'", wildcard_name.to_escaped_for_sql_like)
       .active
       .where("tags.name NOT LIKE ? ESCAPE E'\\\\'", wildcard_name.to_escaped_for_sql_like)
-      .where("tag_aliases.post_count > 0")
-      .order("tag_aliases.post_count desc")
+      .where("tags.post_count > 0")
+      .order("tags.post_count desc")
       .limit(n)
       .pluck(:name, :post_count, :category, :antecedent_name)
       .map {|row| Result.new(*row, :alias)}

test/models/tag_autocomplete_test.rb

@@ -100,7 +100,7 @@ class TagAutocompleteTest < ActiveSupport::TestCase
       ]
       as_user do
         @aliases = [
-          create(:tag_alias, antecedent_name: "/abc", consequent_name: "abcdef", status: "active", post_count: 1)
+          create(:tag_alias, antecedent_name: "/abc", consequent_name: "abcdef", status: "active")
         ]
       end
     end