wibox: Remove some dead code

widget_at() no longer exists since 0aa4304bda (and the surrounding commits
stopped us using this function).

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/drawable.lua

@@ -46,9 +46,6 @@ local function get_widget_context(self)
             screen = s,
             dpi = dpi,
             drawable = self,
-            widget_at = function(_, ...)
-                self:widget_at(...)
-            end
         }
         for k, v in pairs(self._widget_context_skeleton) do
             context[k] = v

lib/wibox/init.lua

@@ -177,9 +177,6 @@ local function new(args)
 
     setup_signals(ret)
     ret.draw = ret._drawable.draw
-    ret.widget_at = function(_, widget, x, y, width, height)
-        return ret._drawable:widget_at(widget, x, y, width, height)
-    end
 
     -- Set the default background
     ret:set_bg(args.bg or beautiful.bg_normal)