Remove clear method in WeakSet and WeakMap

[falsandtru]

Fixes #6500

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[DanielRosenwasser]

@falsandtru as mentioned in the README, you shouldn't be editing files in lib. These changes need to be applied in src/lib, and no changes should be made in lib itself.

[falsandtru]

All right, I fixed it.

[DanielRosenwasser]

👍