saved searches: remove option to disable saved searches.

Remove `SavedSearch.enabled?` checks. There's no need to make saved
searches optional, since Redis is now required to run Danbooru.

test/unit/post_test.rb

@@ -2256,7 +2256,6 @@ class PostTest < ActiveSupport::TestCase
 
     context "saved searches" do
       setup do
-        SavedSearch.stubs(:enabled?).returns(true)
         @post1 = FactoryBot.create(:post, tag_string: "aaa")
         @post2 = FactoryBot.create(:post, tag_string: "bbb")
         FactoryBot.create(:saved_search, query: "aaa", labels: ["zzz"], user: CurrentUser.user)

test/unit/tag_alias_test.rb

@@ -126,10 +126,6 @@ class TagAliasTest < ActiveSupport::TestCase
     end
 
     context "saved searches" do
-      setup do
-        SavedSearch.stubs(:enabled?).returns(true)
-      end
-
       should "move saved searches" do
         tag1 = FactoryBot.create(:tag, :name => "...")
         tag2 = FactoryBot.create(:tag, :name => "bbb")