message: Remove mutex dependency which is not used anymore.
  GitOrigin-RevId:
diff: #ifndef TINK_KEYSET_MANAGER_H_
  #define TINK_KEYSET_MANAGER_H_
  -#include <mutex> // NOLINT(build/c++11)
  -
  #include "absl/base/thread_annotations.h"
  #include "absl/synchronization/mutex.h"
  #include "tink/util/status.h"
  