Remove unused string header

Removed unnecessary string include from treeCreatorDstarSpinAlignMixing.cxx

Author: Mingze129

PWGHF/D2H/TableProducer/treeCreatorDstarSpinAlignMixing.cxx

@@ -46,7 +46,6 @@
 #include <TRandom3.h>
 
 #include <cstdint>
-#include <string>
 #include <vector>
 
 using namespace o2;