Remove the last vestiges of the external allocation cruft.

"Missed a bit". The underlying code has gone (though the
interface is still there for now, backed by a do-nothing implementation).

Bug: 2604080
Change-Id: Iba64d1c193819560423285334dc08015fbc105b1
tree: 4b825dc642cb6eb9a060e54bf8d69288fbee4904

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