ANDROID: staging: ion: Delete unused function ion_heap_buffer_zero

It was deleted in 5d415939(staging: ion: refactor ion's heap
manipulators into a separate file) but was accidentally added back in
a64320f5(Merge 94709049).

Fixes: a64320f5 ("Merge 94709049 ("Merge branch 'akpm' (patches from Andrew)") into android-mainline")

Change-Id: I125d2a78cf4872c80391888e6cd6e10d359d20fa
Signed-off-by: Hridya Valsaraju <hridya@google.com>