Fix a typo

[katoue]

Summary

• PairingHeap<T>::remove(nodeptr_t) called node->Unlink; (missing parentheses), which just formed an unused pointer-to-member-function expression instead of actually calling Unlink(). The node was never detached from its parent's child/sibling chain before being deleted, leaving a dangling pointer in the tree.
• Changed to node->Unlink(); to match the correct call already used elsewhere (e.g. remove_min()'s use of DetachChildren).

Test plan

• CI build/tests pass

[ccpgames-carbon]

All committers have signed the CLA.