Drop the dead multi_cumsum alias

It was only ever assigned the "install another implementation" placeholder in
the branch taken when NumPy is missing; no multi_cumsum exists anywhere in the
package, so it never shows up when NumPy is installed either.

Author: ml31415

numpy_groupies/__init__.py

@@ -13,7 +13,7 @@ def dummy_no_impl(*args, **kwargs):
     import numpy as np
 except ImportError:
     aggregate_np = aggregate_ufunc = dummy_no_impl
-    multi_arange = multi_cumsum = label_contiguous_1d = dummy_no_impl
+    multi_arange = label_contiguous_1d = dummy_no_impl
 else:
     from .aggregate_numpy import aggregate