kernel: Revert UserTimer deletion changes.

This reverts a small portion of f5842905e3.

I added these asserts instead of the deletions to hedge against
the Team/Thread reaching this point with timers still active.
But it seems that doesn't happen, while if thread init fails
due to OOM, we have the default timers still present, which need
to be deleted.

Fixes #20154 and #20148.

src/system/kernel/team.cpp

@@ -520,7 +520,7 @@ Team::~Team()
 	delete_owned_ports(this);
 	sem_delete_owned_sems(this);
 
-	ASSERT(fUserTimers.IsEmpty());
+	DeleteUserTimers(false);
 
 	fPendingSignals.Clear();
 

src/system/kernel/thread.cpp

@@ -336,7 +336,7 @@ Thread::~Thread()
 	if (user_stack_area >= 0)
 		delete_area(user_stack_area);
 
-	ASSERT(fUserTimers.IsEmpty());
+	DeleteUserTimers(false);
 
 	// delete the resources, that may remain in either case