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.

Kernel/Net/E1000NetworkAdapter.cpp

@@ -150,7 +150,7 @@ void E1000NetworkAdapter::detect()
         if (id != qemu_bochs_vbox_id)
             return;
         u8 irq = PCI::get_interrupt_line(address);
-        (void)adopt(*new E1000NetworkAdapter(address, irq)).leak_ref();
+        [[maybe_unused]] auto& unused = adopt(*new E1000NetworkAdapter(address, irq)).leak_ref();
     });
 }