Thread-safe prediction by making the prediction cache thread-local. (#5853)

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

include/xgboost/predictor.h

@@ -55,7 +55,6 @@ struct PredictionCacheEntry {
 class PredictionContainer {
   std::unordered_map<DMatrix *, PredictionCacheEntry> container_;
   void ClearExpiredEntries();
-  std::mutex cache_lock_;
 
  public:
   PredictionContainer() = default;