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/sessionMode.js

@@ -45,7 +45,7 @@ const _modes = {
         unlockDialog: imports.gdm.loginDialog.LoginDialog,
         components: ['polkitAgent'],
         panel: {
-            left: ['logo'],
+            left: [],
             center: ['dateMenu'],
             right: ['a11yGreeter', 'display', 'keyboard',
                     'volume', 'battery', 'powerMenu']