Revert "[ASLayoutSpec] Use childrenMap directly to prevent creating an NSArray within ASDK Part 2 (#2021)"

This reverts commit d5a7c19522.

AsyncDisplayKitTests/ASLayoutSpecSnapshotTestsHelper.m

@@ -42,23 +42,6 @@
   ASSnapshotVerifyNode(node, identifier);
 }
 
-- (void)testFastEnumeration
-{
-  ASLayoutSpec *layoutSpec = [[ASLayoutSpec alloc] init];
-
-  NSMutableArray *children = [NSMutableArray array];
-  for (int i = 0; i < 100; i++) {
-    [children addObject:[[ASDisplayNode alloc] init]];
-  }
-  layoutSpec.children = children;
-  
-  NSInteger i = 0;
-  for (ASDisplayNode *child in layoutSpec) {
-    XCTAssertEqualObjects(child, children[i]);
-    i++;
-  }
-}
-
 @end
 
 @implementation ASTestNode