mtd-tests: flash_speed: Drop an apparently useless block

mtd-tests: flash_speed: Drop an apparently useless block

I know no device without a multiple of 2 number of pages in each
block. Even though it might be the case, it is clearly not a big deal
and we don't really care about reading the last page, we are doing a
speed benchmark; so as long as the throughput calculation knows how much
data has been read it's fine. Eitherway, I don't think we ever have
fallen in this block because we would read the content of two pages (so
one past the block) and put it in a page-wide buffer, which would
probably lead to an out-of-bound abort.

Just drop the block.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>