COMP: Fix one more vxl_config_macros check broken by refactoring

In commit f910d5e7 (Use if(DEFINED) to simplify conditions, 2014-09-08)
we removed sevaral uses of implicit variable evaluation.  However, the
CHECK_TYPE_EXISTS macro 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.

Change-Id: I801bd9daab63b73b30d2d639e9f2d52b3e0b534b
Merge topic 'simplify-if-conditions'

6c56c8bd COMP: Fix one more vxl_config_macros check broken by refactoring