mwan3: check removed route before removal

mwan3: check removed route before removal

This makes mwan3rtmon check if mwan3_get_routes returns a route
before removing it. This helps with IPv6 routes with source address
selector removal where multiple original routes are transformed to
the same mwan3 route if one of the source routes is removed while
the others are kept.

Signed-off-by: Jonas Lochmann <redacted>