Kernel: Remove Process::any_thread()

This was a holdover from the old times when each Process had a special
main thread with TID 0. Using it was a total crapshoot since it would
just return whichever thread was first on the process's thread list.

Now that I've removed all uses of it, we don't need it anymore. :^)

Kernel/Process.cpp

@@ -4612,17 +4612,6 @@ int Process::sys$profiling_disable(pid_t pid)
     return 0;
 }
 
-Thread& Process::any_thread()
-{
-    Thread* found_thread = nullptr;
-    for_each_thread([&](auto& thread) {
-        found_thread = &thread;
-        return IterationDecision::Break;
-    });
-    ASSERT(found_thread);
-    return *found_thread;
-}
-
 WaitQueue& Process::futex_queue(i32* userspace_address)
 {
     auto& queue = m_futex_queues.ensure((FlatPtr)userspace_address);