MTD: sa11x0: Remove shutdown handler

Commit c4a9f88d ([MTD] [NOR] fix ctrl-alt-del can't reboot for
intel flash bug) interferes with this work-around, causing MTD to
issue this warning:

	Flash device refused suspend due to active operation (state 0)

The commit makes our work-around in the map driver unnecessary, so
let's remove it.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>