[OpenMP, test] Fix use of undef VAR_PRIV FileCheck var

Remove the CHECK-NOT directive referring to as-of-yet undefined VAR_PRIV
variable since the pattern of the following CHECK-NOT in the same
CHECK-NOT block covers a superset of the case caught by the first
CHECK-NOT.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D99775