index: create function for deleting node from nodetree
A node is marked deleted by nt_insert(self, node, -1), but "-1" could
mean so many things (notably nullrev), so let's hide it in a
function. The function could possibly also remove nodes from the tree
in the future (it currently only marks them deleted).
Differential Revision: https://phab.mercurial-scm.org/D4004
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