dts: bindings: Remove duplicate in partition.yaml

The label property of the fixed-partitions child binding was duplicated
with two different values. This is invalid yaml, but went unnoticed by
pyYAML. Removed first entry to preserve value produced by pyYAML
behavior of overwriting duplicates.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>