Second parameter of ProxyBuilder::build() is never used and broken

Problem/Motivation

Detected in #3178534: Start running PHPStan on Drupal core (level 0) with PHPStan:

 ------ ------------------------------------------------------- 
  Line   ProxyBuilder/ProxyBuilder.php                          
 ------ ------------------------------------------------------- 
  169    Variable $proxy_class_shortname might not be defined.  
 ------ ------------------------------------------------------- 

It turns our that the second parameter $proxy_class_name is never used and buggy. If you pass a proxy class name "Foo" then you get the exception "ReflectionException: Class Foo does not exist". We cannot use reflection on a class that is about to be generated.

Steps to reproduce

Run PHPStan level 1 on Drupal core.

Proposed resolution

Remove the second parameter of ProxyBuilder::build() completely because it is unused and broken. This is a small API change, but should not affect callers (you can pass more parameters than necessary in PHP).

Remaining tasks

Confirm approach.

API changes

A method parameter is removed. ProxyBuilder is a utility class, not sure if anybody would extend that class and would have to update their method definition?