Remove impl PinCoerceUnsized for Pin

[Darksonn]

The safety requirements for PinCoerceUnsized are essentially that the type does not have a malicious Deref or DerefMut impl. However, the Pin type is fundamental, so the end-user can provide their own implementation of DerefMut for Pin<&SomeLocalType>, so it's possible for Pin to have a malicious DerefMut impl.

Thus, remove the unsound implementation of PinCoerceUnsized for the Pin type.

This PR is a breaking change, but it fixes #85099.

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[Darksonn]

Note that all other fundamental types that implement PinCoerceUnsized prevent this unsoundness by having an unconditional impl of either !DerefMut or DerefMut, both of which prevent the end user from providing an unsound impl of DerefMut.

Note that this also means that making Arc or Rc fundamental would be unsound unless an impl of !DerefMut is added.