mtd: maps: rbtx4939-flash: delete unneeded test

Err has only been initialized to 0 at this, so it is not possible that this
test can be true.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>