C++: Remove test for unrelated issue

The issue for that test is being tested and fixed on PR #2686. Adding a
test here will cause a semantic merge conflict.

cpp/ql/test/library-tests/dataflow/DefaultTaintTracking/defaulttainttracking.cpp

@@ -10,8 +10,8 @@ void sink(const char *);
 void sink(int);
 
 int main(int argc, char *argv[]) {
-  int taintedInt = atoi(getenv("VAR"));
-  taintedInt++; // BUG: `taintedInt` isn't marked as tainted. Only `++` is.
+
+
 
   sink(_strdup(getenv("VAR")));
   sink(strdup(getenv("VAR")));