Nit: remove unused include

Author: pitrou

cpp/src/arrow/filesystem/s3fs.cc

@@ -78,7 +78,6 @@
 #include "arrow/util/checked_cast.h"
 #include "arrow/util/future.h"
 #include "arrow/util/logging.h"
-#include "arrow/util/mutex.h"
 #include "arrow/util/optional.h"
 #include "arrow/util/task_group.h"
 #include "arrow/util/thread_pool.h"