Remove unncessary else block from test (#1325)

Summary:
Just realized #1284 had an extraneous `else` block. This just cleans that up

## Changes in this pull request

See above

### Checklist

- [X] All tests pass. Demo project builds and runs.
- [X] I added tests, an experiment, or detailed why my change isn't tested.
- [ ] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [X] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Pull Request resolved: https://github.com/Instagram/IGListKit/pull/1325

Reviewed By: timonus

Differential Revision: D15511564

Pulled By: timonus

fbshipit-source-id: c21213ee6ccc06ffb0b646381cba3faff0347144

Tests/IGListAdapterTests.m

@@ -738,8 +738,6 @@
 
     if (@available(iOS 11.0, *)) {
         self.collectionView.contentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentNever;
-    } else {
-        // Fallback on earlier versions
     }
     
     // no insets