Revert "mtd: ubi: Fill read disturb statistics"

This reverts commit fae74b62.

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.

Conflicts and compilation errors were addressed.

Change-Id: I129fa63bdbffa6c1881f1a4e702a443741b41d5f
Signed-off-by: Tanya Brokhman <tlinder@codeaurora.org>