remove gcc_unused

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/command/NeighborCommands.cxx

@@ -34,7 +34,7 @@ neighbor_commands_available(const Instance &instance) noexcept
 }
 
 CommandResult
-handle_listneighbors(Client &client, gcc_unused Request args, Response &r)
+handle_listneighbors(Client &client, [[maybe_unused]] Request args, Response &r)
 {
 	const NeighborGlue *const neighbors =
 		client.GetInstance().neighbors.get();