validator suspicious kv-combination for traffic signs

[dieterdreist]

Validator finds nodes tagged "traffic_sign=maxspeed, maxspeed=xy" suspicious by default, but it is a common way to map speed limit signs in my area (there is even a mappaint style). maxspeed on a node with traffic_sign - key present should not be suspicious.

[simon04]

In 5336/josm:

fix #7478 - allow maxspeed= on traffic_sign=maxspeed