Make Project's only_mirror_protected_branches value false by default

This `default_value_for` entry didn't actually do anything because by
default, the UI initialized the flag to `false` and the DB allows `nil`
values.  However, this ensures the model is consistent with that
behavior and avoids the pitfalls we saw in
https://gitlab.com/gitlab-org/gitlab-ce/issues/53515.

This is similar to the change made in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23410 for
remote mirrors.

ee/app/models/ee/project.rb

@@ -100,7 +100,6 @@ module EE
       end
 
       default_value_for :packages_enabled, true
-      default_value_for :only_mirror_protected_branches, true
 
       delegate :store_security_reports_available?, to: :namespace
     end

ee/spec/services/projects/update_mirror_service_spec.rb

@@ -44,6 +44,12 @@ describe Projects::UpdateMirrorService do
       expect(result[:status]).to eq(:success)
     end
 
+    it "disables mirroring protected branches only by default" do
+      new_project = create(:project, :repository, :mirror, import_url: Project::UNKNOWN_IMPORT_URL)
+
+      expect(new_project.only_mirror_protected_branches).to be_falsey
+    end
+
     context "updating tags" do
       it "creates new tags" do
         stub_fetch_mirror(project)