Revert "mtd: ubi: Read threshold verification"

This reverts commit ee81602c.

This patch is part of the "read-disturb implementation" set of patches.
It was decided to take different approach in addressing the read disturb,
thus the whole patch series is reverted in order to simplify the code and
reduce memory footprint.

Change-Id: I90656555a5e8b1ddb9fdc3d0eaa0acff8dbf77eb
Signed-off-by: Tanya Brokhman <tlinder@codeaurora.org>