Remove unnecessary del local variables at end of Gcf.destroy.

`del manager, num` made sense prior to 9f3861f when we used to then call
gc.collect(1) (so we needed to delete the variables from the local frame
so that they can be gc'ed if needed), but we have since then removed the
explicit gc call; thus the del statement doesn't really do anything
anymore (the local frame is about to be destroyed and thus all local
variables will be decref'ed anyways).

Author: anntzer

lib/matplotlib/_pylab_helpers.py

@@ -64,7 +64,6 @@ def destroy(cls, num):
         if hasattr(manager, "_cidgcf"):
             manager.canvas.mpl_disconnect(manager._cidgcf)
         manager.destroy()
-        del manager, num
 
     @classmethod
     def destroy_fig(cls, fig):