mtd: lart: cleanup: drop HAVE_PARTITIONS

Consolidate knowledge about partitions in drivers/mtd/devices/lart.c.
Drop all HAVE_PARTITIONS conditionals. Always use partitioning.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>