fix: remove units parameter from valhalla's isochrones method

It isn't used and has no reason to remain since 'units' has
been removed from directions and matrix methods

Author: mthh

routingpy/routers/valhalla.py

@@ -283,7 +283,6 @@ def isochrones(  # noqa: C901
         generalize: Optional[float] = None,
         preference: Optional[str] = None,
         options: Optional[dict] = None,
-        units: Optional[str] = None,
         language: Optional[str] = None,
         directions_type: Optional[str] = None,
         avoid_locations: Optional[List[List[float]]] = None,