Fix clearing output references with %reset out (#14133)

This should fix #14129. Namely the second reference to the output is
stored in `user_ns_hidden`. That was already cleared during hard
`%reset` but was not cleared when using `%reset out`. Manual
`gc.collect()` is still required.

IPython/core/displayhook.py

@@ -296,9 +296,14 @@ def flush(self):
 
         for n in range(1,self.prompt_count + 1):
             key = '_'+repr(n)
+            try:
+                del self.shell.user_ns_hidden[key]
+            except KeyError:
+                pass
             try:
                 del self.shell.user_ns[key]
-            except: pass
+            except KeyError:
+                pass
         # In some embedded circumstances, the user_ns doesn't have the
         # '_oh' key set up.
         oh = self.shell.user_ns.get('_oh', None)