mtd: Remove unused symbol CONFIG_MTDRAM_ABS_POS

This has been unused, except as the condition for a fatal error, since
commit c13cbf3b ("[MTD] mtdram: Quick cleanup of the driver:") in
2.6.13 (!).

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>