Revert "[clang][dataflow] Expose simple access to child StorageLocation presence." (llvm#145710)

Reverts llvm#145520

Exposed function is no longer needed and side-stepped the intended
contract that the present children are the same set returned by
`getModeledFields()` and presence shouldn't need to be queried for
arbitrary fields.

Author: bazuzi

clang/include/clang/Analysis/FlowSensitive/StorageLocation.h

@@ -168,8 +168,6 @@ class RecordStorageLocation final : public StorageLocation {
     return {Children.begin(), Children.end()};
   }
 
-  bool hasChild(const ValueDecl &D) const { return Children.contains(&D); }
-
 private:
   FieldToLoc Children;
   SyntheticFieldMap SyntheticFields;