tidy: Remove no longer needed np-special casing

np.astype became available in numpy 2.1

Author: danra

xarray/core/duck_array_ops.py

@@ -246,9 +246,6 @@ def astype(data, dtype, *, xp=None, **kwargs):
     if xp is None:
         xp = get_array_namespace(data)
 
-    if xp == np:
-        # numpy currently doesn't have a astype:
-        return data.astype(dtype, **kwargs)
     return xp.astype(data, dtype, **kwargs)