mtd: docg3: remove unused function

As the MTD api has no use for the number of erase cycles
each block has endured, remove the function which calculated
that value.

If one day MTD api finds it usefull for wear levelling
algorithms to have this information, the function should be
put back in place.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>