Revert "Kernel: Make sure threads which don't do any syscalls are t..."

This reverts commit 3c3a172.

We cannot blindly kill threads just because they're not executing in a
system call. Being blocked (including in a page fault) needs proper
unblocking and potentially kernel stack cleanup before we can mark a
thread as Dying.

Fixes SerenityOS#8691

Kernel/Scheduler.cpp

@@ -217,13 +217,6 @@ bool Scheduler::pick_next()
 
  ScopedSpinLock lock(g_scheduler_lock);
 
- auto current_thread = Thread::current();
- if (current_thread->should_die() && current_thread->may_die_immediately()) {
- // Ordinarily the thread would die on syscall exit, however if the thread
- // doesn't perform any syscalls we still need to mark it for termination here.
- current_thread->set_state(Thread::Dying);
- }
-
  if constexpr (SCHEDULER_RUNNABLE_DEBUG) {
  dump_thread_list();
  }

Kernel/Syscall.cpp

@@ -4,7 +4,6 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/ScopeGuard.h>
 #include <Kernel/API/Syscall.h>
 #include <Kernel/Arch/x86/Interrupts.h>
 #include <Kernel/Arch/x86/TrapFrame.h>
@@ -154,14 +153,6 @@ NEVER_INLINE void syscall_handler(TrapFrame* trap)
 {
  auto& regs = *trap->regs;
  auto current_thread = Thread::current();
- {
- ScopedSpinLock lock(g_scheduler_lock);
- current_thread->set_may_die_immediately(false);
- }
- ScopeGuard reset_may_die_immediately = [&current_thread] {
- ScopedSpinLock lock(g_scheduler_lock);
- current_thread->set_may_die_immediately(true);
- };
  VERIFY(current_thread->previous_mode() == Thread::PreviousMode::UserMode);
  auto& process = current_thread->process();
 

Kernel/Thread.h

@@ -1187,8 +1187,6 @@ class Thread
  bool is_profiling_suppressed() const { return m_is_profiling_suppressed; }
  void set_profiling_suppressed() { m_is_profiling_suppressed = true; }
 
- bool may_die_immediately() const { return m_may_die_immediately; }
- void set_may_die_immediately(bool flag) { m_may_die_immediately = flag; }
  InodeIndex global_procfs_inode_index() const { return m_global_procfs_inode_index; }
 
 private:
@@ -1287,7 +1285,6 @@ class Thread
  Kernel::Lock* m_blocking_lock { nullptr };
  u32 m_lock_requested_count { 0 };
  IntrusiveListNode<Thread> m_blocked_threads_list_node;
- bool m_may_die_immediately { true };
 
 #if LOCK_DEBUG
  struct HoldingLockInfo {