remove explanation.reset() and fixes in add_var_bound()

Signed-off-by: Lev <levnach@hotmail.com>

src/util/lp/explanation.h

@@ -34,7 +34,7 @@ public:
         m_set_of_ci.insert(j);
         m_explanation.push_back(std::make_pair(one_of_type<mpq>(), j));
     }
-    void reset() { m_explanation.reset(); }
+
     template <typename A> void add(const A& a) { for (constraint_index j : a) push_justification(j); }
 };
 }