Uninstall default bottle plugins

By default, bottle installs two plugins: `JSONPlugin` and `TemplatePlugin`.

`TemplatePlugin` is always a no-op, because none of the routes we use
(like `/ready`) have a template parameter.

`JSONPlugin` could have been useful were it actually serializing all json
messages. Instead it only deals with dictionaries. Ycmd has always had
`ycmd.handlers._JsonResponse` and we will need to keep it that way.
`_JsonResponse` makes `JSONPlugin` a no-op too, because, by the time we get
to the `JSONPlugin.apply()` the response is always a str already.

`Bottle.uninstall()` can be passed:

1. An instance of an installed plugins, to uninstall that one plugin.
2. A type, to uninstall all plugins of that type.
3. `True`, to uninstall all plugins.

Since we are uninstalling all default plugins, we can just do

`handlers.app.uninstall( True )` and then install the plugins we care
about - `ycmd.HmacPlugin` and `ycmd.WatchdogPlugin`.

ycmd/__main__.py

@@ -203,6 +203,7 @@ def Main():
                                     args.stderr,
                                     args.keep_logfiles )
   atexit.register( handlers.ServerCleanup )
+  handlers.app.uninstall( True )
   handlers.app.install( WatchdogPlugin( args.idle_suicide_seconds,
                                         args.check_interval_seconds ) )
   handlers.app.install( HmacPlugin( hmac_secret ) )