core: exclude CorePlugin from plugins_listing

Summary:
The `plugins_listing` response has long included a `"core"` key. Prior
to #2299 and #2304, this did nothing; now, it properly emits a warning,
because the frontend does not know how to render such a plugin. There’s
no reason that the frontend needs to know about this plugin at all.

Test Plan:
Observe that the “Plugin has no loading mechanism and no baked-in
registry entry: core” warning no longer appears in the JS console.

wchargin-branch: core-verb

tensorboard/backend/application.py

@@ -277,6 +277,9 @@ def _serve_plugins_listing(self, request):
     """
     response = collections.OrderedDict()
     for plugin in self._plugins:
+      if type(plugin) is core_plugin.CorePlugin:  # pylint: disable=unidiomatic-typecheck
+        # This plugin's existence is a backend implementation detail.
+        continue
       start = time.time()
       is_active = plugin.is_active()
       elapsed = time.time() - start