mtd: nand: omap: remove is_elm_present flag

'is_elm_present' flag is not used anywhere. This check is implicitely
taken care while selecting appropriate ecc-scheme via DT or board-file.

Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>