Kernel: Stop eagerly loading entire executables

We were forced to do this because the page fault code would fall apart
when trying to generate a backtrace for a non-current thread.

This issue has been fixed for a while now, so let's go back to lazily
loading executable pages which should make everything a little better.

Kernel/Process.cpp

@@ -361,10 +361,6 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
         ASSERT(success);
     }
 
-#ifdef EXPENSIVE_USERSPACE_STACKS
-    region->page_in();
-#endif
-
     OwnPtr<ELFLoader> loader;
     {
         // Okay, here comes the sleight of hand, pay close attention..