[libc++][NFC] Move __memory/aligned_alloc.h into src/ (#166172)

This header is only ever used inside `src/`, so we might as well move it
there. As a drive-by this also removes some dead code.

NOKEYCHECK=True
GitOrigin-RevId: 3dcdb4c7658fb955d61fde5bd5232bdeadfc7eeb