Remove references to the "unstable bits" issue

Remove references to the "unstable bits" issue

In 2011, several sections about an issue with SLC and MLC NAND flashes of
that time were added. The problem was referred to as "unstable bits" issue
throughout the documentation and the FAQ.

As of writing, the issue could not be reproduced with modern NAND flashes
for several years and the section on the web site actually caused some
confusion, with some users of MTD/UBI/UBIFS attributing actual bugs to the
unstable bits issue instead of reporting on the mailing list.

To avoid future confusion, this patch removes the section about the issue
and all references to it.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>