Removed unused let blocks (#7446)

The bodies of these blocks were never evaluated.

spec/serializers/basic_group_serializer_spec.rb

@@ -31,12 +31,6 @@ describe BasicGroupSerializer do
     end
 
     describe 'group owner' do
-      let(:user) do
-        user = Fabricate(:user)
-        group.add_owner(user)
-        user
-      end
-
       it 'should include bio_raw' do
         expect(subject.as_json[:bio_raw]).to eq('testing')
       end