consolidate cancellation to context check_cancel_flag instead of calling in set_cancel()

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/smt/smt_model_checker.cpp

@@ -40,7 +40,6 @@ namespace smt {
         m_max_cexs(1),
         m_iteration_idx(0),
         m_curr_model(0),
-        m_cancel(false),
         m_new_instances_bindings(m) {
     }