disable lookahead simplification when external solver is used

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

src/sat/sat_lookahead.cpp

@@ -2317,7 +2317,7 @@ namespace sat {
                 }                
             }
         }   
-        m_lookahead.reset();        
+        m_lookahead.reset();    
     }