Remove failedLow from the mainThread struct

failedLow is in fact a local variable in Thread::search().

Also clean some spaces and tabs in code.

No functional change.

src/thread.h

@@ -85,7 +85,6 @@ struct MainThread : public Thread {
   void search() override;
   void check_time();
 
-  bool failedLow;
   double bestMoveChanges, previousTimeReduction;
   Value previousScore;
   int callsCnt;