Attributes: Shave off a couple of bytes

The `attrHooks` entries for boolean attributes are only defined for jQuery 4+;
jQuery 3.x used a separate mechanism - assigning them to
`jQuery.expr.attrHandle`. That object used to be maintained by Sizzle, since
jQuery 3.7.0 it's kept in the selector module. Because of that, the `isXMLDoc`
check used to be require in this hook.

Now that standard `attrHooks` are used, the `isXMLDoc` check already happens
inside of `jQuery.attr` and there's no need to repeat it in the test. Note that
this repetition is even incorrect - while Sizzle's `jQuery.find.attr` used to
treat an `undefined` output of the hooks from `jQuery.expr.attrHandle` as a way
to opt out of the hook, jQuery's `attrHooks` use `null` to opt out of a getter
hook.

Apart from the size, this patch also avoids unnecessary extra checks.

Closes gh-5398

src/attributes/attr.js

@@ -108,16 +108,9 @@ jQuery.each( (
 ).split( " " ), function( _i, name ) {
 	jQuery.attrHooks[ name ] = {
 		get: function( elem ) {
-			var ret,
-				isXML = jQuery.isXMLDoc( elem ),
-				lowercaseName = name.toLowerCase();
-
-			if ( !isXML ) {
-				ret = elem.getAttribute( name ) != null ?
-					lowercaseName :
+			return elem.getAttribute( name ) != null ?
+				name.toLowerCase() :
 				null;
-			}
-			return ret;
 		},
 
 		set: function( elem, value, name ) {