MNT: Use Gcf.destroy(manager) instead of Gcf.destroy(manager.num)

Going through manager.num is unnecessary as destroy() only looks up
the manager for the given number again.

As the docstring of destroy() explains:

> It is recommended to pass a manager instance, to avoid confusion when
two managers share the same number.

Author: timhoffm

lib/matplotlib/_pylab_helpers.py

@@ -68,10 +68,10 @@ def destroy(cls, num):
     @classmethod
     def destroy_fig(cls, fig):
         """Destroy figure *fig*."""
-        num = next((manager.num for manager in cls.figs.values()
-                    if manager.canvas.figure == fig), None)
-        if num is not None:
-            cls.destroy(num)
+        manager = next((manager for manager in cls.figs.values()
+                       if manager.canvas.figure == fig), None)
+        if manager is not None:
+            cls.destroy(manager)
 
     @classmethod
     def destroy_all(cls):