Add dbg_clear helper function

closes https://github.com/official-stockfish/Stockfish/pull/5921

No functional change

src/misc.h

@@ -73,6 +73,7 @@ void dbg_stdev_of(int64_t value, int slot = 0);
 void dbg_extremes_of(int64_t value, int slot = 0);
 void dbg_correl_of(int64_t value1, int64_t value2, int slot = 0);
 void dbg_print();
+void dbg_clear();
 
 using TimePoint = std::chrono::milliseconds::rep;  // A value in milliseconds
 static_assert(sizeof(TimePoint) == sizeof(int64_t), "TimePoint should be 64 bits");