Remove broken CSS for ve.ui.MWReferenceGroupInputWidget

This CSS exists since I2ab47e7 from August 2014. The original idea
was to dim the default "General references" when you edit a <ref> or
<references> list in VisualEditor.

Steps to reproduce:
* Start VisualEditor.
* Edit a <ref> or <references> list.
* Edit the group.
* You will see the dimmed text "General references". This is not the
  CSS in this patch, but the default styling for OOUI placeholders.
* Open the dropdown. The list will show a "General references" item.
  It's not dimmed. This is where the CSS was meant to be.

The CSS class name in the OOUI mixin was actually changed from
"oo-ui-flaggableElement-…" to "oo-ui-flaggedElement-…" via I1abecd8,
just a few days later.

In addition the selector wouldn't work anyway for other reasons.
The dropdown is not inside the `.ve-ui-mwReferenceGroupInputWidget`
container any more but placed outside by the OOUI window manager.
And the selector's specifity is to low, at least since Ic57b3ff.

I argue it's not worth fixing it. Nobody missed it for 10 years.
Light gray text would be illegible anyway on the light gray/light
blue backgrounds used in the dropdown menu. Let's consider it dead
code and just remove it.

The class name doesn't appear anywhere else (any more):
https://codesearch.wmcloud.org/search/?q=flaggableElement

Change-Id: Ia802303737ba35cd4b14fae924b7227472f905fd

extension.json

@@ -116,7 +116,6 @@
 			"styles": [
 				"ve.ui.MWReferenceDialog.less",
 				"ve.ui.MWReferenceContextItem.less",
-				"ve.ui.MWReferenceGroupInputWidget.less",
 				"ve.ui.MWReferenceResultWidget.less",
 				"ve.ui.MWReferenceSearchWidget.less",
 				"ve.ui.MWCitationDialogTool.less"

modules/ve-cite/ve.ui.MWReferenceGroupInputWidget.js

@@ -43,8 +43,7 @@ OO.inheritClass( ve.ui.MWReferenceGroupInputWidget, OO.ui.ComboBoxInputWidget );
 ve.ui.MWReferenceGroupInputWidget.prototype.populateMenu = function ( internalList ) {
 	const items = [ new OO.ui.MenuOptionWidget( {
 		data: '',
-		label: this.emptyGroupName,
-		flags: 'emptyGroupPlaceholder'
+		label: this.emptyGroupName
 	} ) ];
 	for ( const groupName in internalList.getNodeGroups() ) {
 		const match = groupName.match( /^mwReference\/(.+)/ );

modules/ve-cite/ve.ui.MWReferenceGroupInputWidget.less

@@ -1,10 +0,0 @@
-/*!
- * VisualEditor MediaWiki UserInterface MWReferenceGroupInputWidget styles.
- *
- * @copyright 2011-2018 VisualEditor Team's Cite sub-team and others; see AUTHORS.txt
- * @license MIT
- */
-
-.ve-ui-mwReferenceGroupInputWidget .oo-ui-flaggableElement-emptyGroupPlaceholder {
-	color: #72777d;
-}