Fix invalid assertions in build details entity specs

spec/factories/ci/builds.rb

@@ -140,7 +140,6 @@ FactoryGirl.define do
     end
     trait :erased do
-      erasable
       erased_at Time.now
       erased_by factory: :user
     end

spec/serializers/build_details_entity_spec.rb

@@ -87,18 +87,18 @@ describe BuildDetailsEntity do
         end
       end
-      context 'when the build has been erased' do
-        let(:build) { create(:ci_build, :erased, project: project) }
-        it 'exposes the user whom erased the build' do
+      context 'when the build has not been erased' do
+        let(:build) { create(:ci_build, :erasable, project: project) }
+        it 'exposes a build erase path' do
           expect(subject).to include(:erase_path)
         end
       end
       context 'when the build has been erased' do
-        let(:build) { create(:ci_build, :erased, project: project, erased_by: user) }
-        it 'exposes the user whom erased the build' do
+        let(:build) { create(:ci_build, :erased, project: project) }
+        it 'exposes the user who erased the build' do
           expect(subject).to include(:erased_by)
         end
       end