Remove unneeded header

Sonny Martin · Sonny Martin · commit e37e9b89a541 · 2018-08-22T12:18:37.000+01:00
diff --git a/src/util/invariant.h b/src/util/invariant.h
@@ -12,7 +12,6 @@ Author: Martin Brain, martin.brain@diffblue.com
 #include <cstdlib>
 #include <stdexcept>
 #include <string>
-#include <tuple>
 #include <type_traits>
 
 /*
