Herwig7_i fix clang10 warnings
fix
/build/atnight/localbuilds/nightlies/Athena/master/athena/Generators/Herwig7_i/src/PowhegLesHouchesFileReader.cxx:33:11: warning: unused variable 'max_num_weights' [-Wunused-const-variable]
const int max_num_weights=100;