remove constexpr if () in test_ad_matvar to be compliant with gcc <= 7.5.0

Author: SteveBronder

test/unit/math/test_ad_matvar.hpp

@@ -448,7 +448,7 @@ inline void expect_ad_matvar_impl(const ad_tolerances& tols, const F& f,
         },
         A_vm_tuple);
 
-    if constexpr (!any_varmat) {
+    if (!any_varmat) {
       SUCCEED();  // If no varmats are created, skip this test
       return;
     } else {