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: I4e8d4f6734967caf4f0d19c734027f7b6c107370