Revert "Revert "ota: Remove cruft we don't care about""

Should prevent users from "downgrading" builds, but instead of checking the
build date of the ROM, updater-script checks the build date of the recovery.
As a result, "downgrading" is still possible, and installation of the ROM,
if the recovery is newer than it, is impossible.

This reverts commit 28032f6b.

Change-Id: I71ce8eb16074079f01dfabcb19ed30e3586c59ac