Removed unused variable (#1188)

Summary:
Issue fixed: #1187

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] 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)
Closes #1188

Differential Revision: D8204493

Pulled By: rnystrom

fbshipit-source-id: 52bf75e6436d95b127d44302c4eb6f0bfdc1da08

Source/IGListAdapter.m

@@ -1080,8 +1080,7 @@ - (__kindof UICollectionReusableView *)dequeueReusableSupplementaryViewOfKind:(N
 - (void)performBatchAnimated:(BOOL)animated updates:(void (^)(id<IGListBatchContext>))updates completion:(void (^)(BOOL))completion {
     IGAssertMainThread();
     IGParameterAssert(updates != nil);
-    UICollectionView *collectionView = self.collectionView;
-    IGAssert(collectionView != nil, @"Performing batch updates without a collection view.");
+    IGAssert(self.collectionView != nil, @"Performing batch updates without a collection view.");
 
     [self _enterBatchUpdates];