loginDialog: Remove logo in upper left corner

With optional branding now being shown below the user list, we can
remove the unloved instance in the upper left corner ...

https://bugzilla.gnome.org/show_bug.cgi?id=694912

js/ui/panel.js

@@ -929,7 +929,6 @@ const PANEL_ITEM_IMPLEMENTATIONS = {
     'volume': imports.ui.status.volume.Indicator,
     'battery': imports.ui.status.power.Indicator,
     'lockScreen': imports.ui.status.lockScreenMenu.Indicator,
-    'logo': imports.gdm.loginDialog.LogoMenuButton,
     'keyboard': imports.ui.status.keyboard.InputSourceIndicator,
     'powerMenu': imports.gdm.powerMenu.PowerMenuButton,
     'userMenu': imports.ui.userMenu.UserMenuButton