Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Libraries/LibPthread/pthread.cpp

@@ -460,19 +460,13 @@ int pthread_attr_setstacksize(pthread_attr_t* attributes, size_t stack_size)
     return 0;
 }
 
-int pthread_getschedparam(pthread_t thread, int* policy, struct sched_param* param)
+int pthread_getschedparam([[maybe_unused]] pthread_t thread, [[maybe_unused]] int* policy, [[maybe_unused]] struct sched_param* param)
 {
-    (void)thread;
-    (void)policy;
-    (void)param;
     return 0;
 }
 
-int pthread_setschedparam(pthread_t thread, int policy, const struct sched_param* param)
+int pthread_setschedparam([[maybe_unused]] pthread_t thread, [[maybe_unused]] int policy, [[maybe_unused]] const struct sched_param* param)
 {
-    (void)thread;
-    (void)policy;
-    (void)param;
     return 0;
 }
 

Libraries/LibPthread/pthread.h

@@ -67,7 +67,7 @@ int pthread_attr_setstack(pthread_attr_t* attr, void*, size_t);
 int pthread_attr_getstacksize(const pthread_attr_t*, size_t*);
 int pthread_attr_setstacksize(pthread_attr_t*, size_t);
 
-int pthread_once(pthread_once_t*, void (*)(void));
+int pthread_once(pthread_once_t*, void (*)());
 #define PTHREAD_ONCE_INIT 0
 void* pthread_getspecific(pthread_key_t key);
 int pthread_setspecific(pthread_key_t key, const void* value);
@@ -100,14 +100,14 @@ int pthread_cancel(pthread_t);
 int pthread_cond_destroy(pthread_cond_t*);
 int pthread_cond_timedwait(pthread_cond_t*, pthread_mutex_t*, const struct timespec*);
 
-void pthread_testcancel(void);
+void pthread_testcancel();
 
 int pthread_spin_destroy(pthread_spinlock_t*);
 int pthread_spin_init(pthread_spinlock_t*, int);
 int pthread_spin_lock(pthread_spinlock_t*);
 int pthread_spin_trylock(pthread_spinlock_t*);
 int pthread_spin_unlock(pthread_spinlock_t*);
-pthread_t pthread_self(void);
+pthread_t pthread_self();
 int pthread_detach(pthread_t);
 int pthread_equal(pthread_t, pthread_t);
 int pthread_mutexattr_init(pthread_mutexattr_t*);

Libraries/LibPthread/pthread_once.cpp

@@ -37,7 +37,7 @@ enum State : i32 {
     PERFORMING_WITH_WAITERS,
 };
 
-int pthread_once(pthread_once_t* self, void (*callback)(void))
+int pthread_once(pthread_once_t* self, void (*callback)())
 {
     auto& state = reinterpret_cast<Atomic<State>&>(*self);