Revert "updater: Fix and improve allowing devices to suppress BLKDISCARD"

The whole point of the flag, was to allow devices with slow secure
wiping to skip it. This is just making it even slower, thus defeating
the flags purpose. Also, this particular code has nothing to do with
regular wiping, but only executed during installing/updating.

This reverts commit 6f54468d.

Change-Id: I5f910520b58ceb20037091bba8a762cca026c22b