mips: tx99: remove CONFIG_MTD_PARTITIONS check

CONFIG_MTD_PARTITIONS no longer exists as MTD partitioning is always
enabled now.

Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>