Revert "Zero blocks before BLKDISCARD"

This reverts commit b65f0272.

It slows down the update too much on some devices (e.g. increased
from 8 mins to 40 mins to take a full OTA update).

Bug: 22129621
Change-Id: I016e3b47313e3113f01bb4f8eb3c14856bdc35e5
(cherry picked from commit 7125f959)