mtd: onenand: omap: remove two unused functions

Nothing calls omap2_onenand_rephase(). And __adjust_timing() is only
called by omap2_onenand_rephase(). Remove these two unused functions.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>