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/StorageCommands.cxx

@@ -143,7 +143,7 @@ print_storage_uri(Client &client, Response &r, const Storage &storage)
 }
 
 CommandResult
-handle_listmounts(Client &client, gcc_unused Request args, Response &r)
+handle_listmounts(Client &client, [[maybe_unused]] Request args, Response &r)
 {
 	Storage *_composite = client.GetInstance().storage;
 	if (_composite == nullptr) {