Disable relation split warning for type=route

[NE2]

This is an extension of https://josm.openstreetmap.de/ticket/6601 (which was closed and reopened, but got no response - do reopened bugs slip through the cracks somehow?). When you split way 35393947, for example, you get a warning about splitting a role based relation. It was fixed for roles of forward and backward, but not the directional roles that are common in the U.S. If there are certain types of route relations where this warning is necessary, adding north/south/east/west to the ignored roles will mostly fix it (but there are other custom roles like north (express)).