fix unused-but-set-variable warnings reported in #579

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

src/interp/iz3translate.cpp

@@ -342,6 +342,7 @@ public:
                     else
                         cls1.push_back(cls2[j]);
                 }
+                (void)found_pivot2;
                 assert(found_pivot2);
                 return;
             }

src/interp/iz3translate_direct.cpp

@@ -362,6 +362,7 @@ public:
                     else
                         cls1.push_back(cls2[j]);
                 }
+                (void)found_pivot2;
                 assert(found_pivot2);
                 return;
             }