ion: remove unused function

Somehow, during [83c50939e88d0 gpu: ion: optimize zero'ing of pages] we
retained some cruft. Remove it.

Change-Id: I38f2ef5d6cd1baa3fe8a758a0bf8164695e2b657
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>