Remove ThreadWorkPool, replace by WorkerThreadPool

The former needs to be allocated once per usage. The later is shared for all threads, which is more efficient.
It can also be better debugged.

servers/rendering_server.cpp

@@ -2831,7 +2831,6 @@ void RenderingServer::set_render_loop_enabled(bool p_enabled) {
 RenderingServer::RenderingServer() {
 	//ERR_FAIL_COND(singleton);
 
-	thread_pool = memnew(RendererThreadPool);
 	singleton = this;
 }
 
@@ -3032,6 +3031,5 @@ void RenderingServer::init() {
 }
 
 RenderingServer::~RenderingServer() {
-	memdelete(thread_pool);
 	singleton = nullptr;
 }