Fix one more vxl_config_macros check broken by refactoring

In commit 9a27bf34 (Simplify vxl_config_macros conditions with
if(DEFINED), 2014-09-08) we removed sevaral uses of implicit variable
evaluation.  However, CHECK_TYPE_EXISTS was abusing details of the prior
implementation and depended on setting a check result variable to its
own name to reset the check.  Use unset() instead.