dts/bindings/mtd: Remove dead definitions from partition binding

The 'partition\d+:' definition might be a leftover from an older way of
handling partitions. There are no 'partition[0-9]' properties anywhere
in the DTS files.

The reason I want to get rid of this is that it's the only place where
you get a 'properties:' nested inside inside a 'properties:'. That's an
undocumented feature, and removing it will make generate_node_defines()
much simpler (non-recursive).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>