Core: Drop the root parameter of jQuery.fn.init

The third parameter of `jQuery.fn.init` - `root` - was just needed to support
`jQuery.sub`. Since this API has been removed in jQuery 1.9.0 and Migrate 3.x
is not filling it in, this parameter is no longer needed.

This parameter has never been documented but it's safer to remove it in a major
update.

Closes gh-5096

src/core/init.js

@@ -15,18 +15,14 @@ var rootjQuery,
 	// Shortcut simple #id case for speed
 	rquickExpr = /^(?:\s*(<[\w\W]+>)[^>]*|#([\w-]+))$/,
 
-	init = jQuery.fn.init = function( selector, context, root ) {
+	init = jQuery.fn.init = function( selector, context ) {
 		var match, elem;
 
 		// HANDLE: $(""), $(null), $(undefined), $(false)
 		if ( !selector ) {
 			return this;
 		}
 
-		// Method init() accepts an alternate rootjQuery
-		// so migrate can support jQuery.sub (gh-2101)
-		root = root || rootjQuery;
-
 		// HANDLE: $(DOMElement)
 		if ( selector.nodeType ) {
 			this[ 0 ] = selector;
@@ -36,8 +32,8 @@ var rootjQuery,
 		// HANDLE: $(function)
 		// Shortcut for document ready
 		} else if ( typeof selector === "function" ) {
-			return root.ready !== undefined ?
-				root.ready( selector ) :
+			return rootjQuery.ready !== undefined ?
+				rootjQuery.ready( selector ) :
 
 				// Execute immediately if ready is not present
 				selector( jQuery );
@@ -108,7 +104,7 @@ var rootjQuery,
 
 			// HANDLE: $(expr) & $(expr, $(...))
 			} else if ( !context || context.jquery ) {
-				return ( context || root ).find( selector );
+				return ( context || rootjQuery ).find( selector );
 
 			// HANDLE: $(expr, context)
 			// (which is just equivalent to: $(context).find(expr)