chore: remove deprecated plugin hook methods [breaking]

Call plugins.hooks.fire() instead.

src/plugins/index.js

@@ -25,13 +25,6 @@ require('./usage')(Plugins);
 Plugins.data = require('./data');
 Plugins.hooks = require('./hooks');
 
-// Backwards compatibility for hooks, remove in v1.17.0
-Plugins.registerHook = Plugins.hooks.register;
-Plugins.unregisterHook = Plugins.hooks.unregister;
-Plugins.fireHook = Plugins.hooks.fire;
-Plugins.hasListeners = Plugins.hooks.hasListeners;
-// end
-
 Plugins.getPluginPaths = Plugins.data.getPluginPaths;
 Plugins.loadPluginInfo = Plugins.data.loadPluginInfo;