Kernel: Remove "stale callback" concept from time management

If a hardware timer doesn't have a callback registered, it's now simply
represented by a null m_callback.

Kernel/Time/HPETComparator.cpp

@@ -34,11 +34,11 @@ namespace Kernel {
 
 NonnullRefPtr<HPETComparator> HPETComparator::create(u8 number, u8 irq, bool periodic_capable)
 {
-    return adopt(*new HPETComparator(number, irq, periodic_capable, [](const RegisterState& regs) { TimeManagement::stale_function(regs); }));
+    return adopt(*new HPETComparator(number, irq, periodic_capable));
 }
 
-HPETComparator::HPETComparator(u8 number, u8 irq, bool periodic_capable, Function<void(const RegisterState&)> callback)
-    : HardwareTimer(irq, move(callback))
+HPETComparator::HPETComparator(u8 number, u8 irq, bool periodic_capable)
+    : HardwareTimer(irq)
     , m_periodic(false)
     , m_periodic_capable(periodic_capable)
     , m_comparator_number(number)

Kernel/Time/HPETComparator.h

@@ -58,7 +58,7 @@ public:
 private:
     void set_new_countdown();
     virtual void handle_irq(const RegisterState&) override;
-    HPETComparator(u8 number, u8 irq, bool periodic_capable, Function<void(const RegisterState&)> callback);
+    HPETComparator(u8 number, u8 irq, bool periodic_capable);
     bool m_periodic : 1;
     bool m_periodic_capable : 1;
     bool m_edge_triggered : 1;

Kernel/Time/HardwareTimer.cpp

@@ -37,7 +37,8 @@ HardwareTimer::HardwareTimer(u8 irq_number, Function<void(const RegisterState&)>
 
 void HardwareTimer::handle_irq(const RegisterState& regs)
 {
-    m_callback(regs);
+    if (!m_callback)
+        m_callback(regs);
 }
 
 const char* HardwareTimer::purpose() const

Kernel/Time/HardwareTimer.h

@@ -62,7 +62,7 @@ public:
     virtual size_t calculate_nearest_possible_frequency(size_t frequency) const = 0;
 
 protected:
-    HardwareTimer(u8 irq_number, Function<void(const RegisterState&)>);
+    HardwareTimer(u8 irq_number, Function<void(const RegisterState&)> = nullptr);
     //^IRQHandler
     virtual void handle_irq(const RegisterState&) override;
     u64 m_frequency { OPTIMAL_TICKS_PER_SECOND_RATE };

Kernel/Time/TimeManagement.cpp

@@ -90,10 +90,6 @@ time_t TimeManagement::boot_time() const
     return RTC::boot_time();
 }
 
-void TimeManagement::stale_function(const RegisterState&)
-{
-}
-
 TimeManagement::TimeManagement(bool probe_non_legacy_hardware_timers)
 {
     if (ACPI::is_enabled()) {

Kernel/Time/TimeManagement.h

@@ -57,7 +57,6 @@ public:
     static void update_time(const RegisterState&);
     void increment_time_since_boot(const RegisterState&);
 
-    static void stale_function(const RegisterState&);
     static bool is_hpet_periodic_mode_allowed();
 
 private: