staging: ion: remove dead code

ion_heap_pages_zero() function is not used at all, so remove it to
simplify the ion_heap_sglist_zero() function later.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20200619103636.11974-30-m.szyprowski@samsung.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>