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.

Applications/SystemMonitor/MemoryStatsWidget.cpp

@@ -105,8 +105,7 @@ void MemoryStatsWidget::refresh()
     ASSERT(json_result.has_value());
     auto json = json_result.value().as_object();
 
-    unsigned kmalloc_eternal_allocated = json.get("kmalloc_eternal_allocated").to_u32();
-    (void)kmalloc_eternal_allocated;
+    [[maybe_unused]] unsigned kmalloc_eternal_allocated = json.get("kmalloc_eternal_allocated").to_u32();
     unsigned kmalloc_allocated = json.get("kmalloc_allocated").to_u32();
     unsigned kmalloc_available = json.get("kmalloc_available").to_u32();
     unsigned user_physical_allocated = json.get("user_physical_allocated").to_u32();