build: Restore failing Phan build on PHP 8.0+

Follows-up e68d82f. The only reason .phan/stubs/ existed
was to replace the Selector.php file with a stub that doesn't cause
Phan's parser to crash on PHP 8.0, as per
phan/phan#4751.

Removing that, brought the fatal back when running `composer phan`
on PHP 8.0+ locally (and also on the CI builds at
https://github.com/wikimedia/less.php).

Change-Id: I913f6b7bc22457aefbac4bbf449dec3e0239d4df

.phan/stubs/Less_Tree_Selector.php

@@ -1,5 +1,7 @@
 <?php
-
+/**
+ * @method int match($other)
+ */
 class Less_Tree_Selector extends Less_Tree {
 	public $elements;
 	public $condition;
@@ -25,8 +27,9 @@ public function accept( $visitor ) {
 	public function createDerived( $elements, $extendList = null, $evaldCondition = null ) {
 	}
 
-	public function match( $other ) {
-	}
+	// https://github.com/phan/phan/issues/4751
+	// public function match( $other ) {
+	// }
 
 	public function CacheElements() {
 	}