mtd: drop select of MTD_PARTITIONS

MTD_PARTITIONS got killed in commit 6a8a98b2. This means that since
v3.0 this Kconfig symbol doesn't exist anymore. Apparently selecting
a non-existant symbol is a nop. Drop that select.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>