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/UDPSocket.cpp

@@ -79,9 +79,8 @@ NonnullRefPtr<UDPSocket> UDPSocket::create(int protocol)
     return adopt(*new UDPSocket(protocol));
 }
 
-KResultOr<size_t> UDPSocket::protocol_receive(ReadonlyBytes raw_ipv4_packet, UserOrKernelBuffer& buffer, size_t buffer_size, int flags)
+KResultOr<size_t> UDPSocket::protocol_receive(ReadonlyBytes raw_ipv4_packet, UserOrKernelBuffer& buffer, size_t buffer_size, [[maybe_unused]] int flags)
 {
-    (void)flags;
     auto& ipv4_packet = *(const IPv4Packet*)(raw_ipv4_packet.data());
     auto& udp_packet = *static_cast<const UDPPacket*>(ipv4_packet.payload());
     ASSERT(udp_packet.length() >= sizeof(UDPPacket)); // FIXME: This should be rejected earlier.