simplify api_ast.cpp: remove dead code and empty comment

- Z3_get_index_value: remove redundant null check on 'va'; after verifying
  _a->get_kind() == AST_VAR, to_var(_a) always returns a valid pointer,
  so the 'if (va)' branch and its error-handling were unreachable dead code.
- Z3_mk_rec_func_decl: remove empty comment line ('//').

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/api/api_ast.cpp

@@ -129,7 +129,6 @@ extern "C" {
         Z3_TRY;
         LOG_Z3_mk_rec_func_decl(c, s, domain_size, domain, range);
         RESET_ERROR_CODE();
-        // 
         recfun::promise_def def = 
             mk_c(c)->recfun().get_plugin().mk_def(
                 to_symbol(s), domain_size, to_sorts(domain), to_sort(range), false);
@@ -1523,11 +1522,7 @@ extern "C" {
             return 0;
         }
         var* va = to_var(_a);
-        if (va) {
-            return va->get_idx();
-        }
-        SET_ERROR_CODE(Z3_INVALID_ARG, nullptr);
-        return 0;
+        return va->get_idx();
         Z3_CATCH_RETURN(0);
     }