Clean up unused includes

Some late spring cleaning ...

js/ui/main.js

@@ -16,7 +16,6 @@ const Lang = imports.lang;
 const Mainloop = imports.mainloop;
 const Meta = imports.gi.Meta;
 const Shell = imports.gi.Shell;
-const Signals = imports.signals;
 const St = imports.gi.St;
 
 const Chrome = imports.ui.chrome;