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

OpenMP test target_data_use_device_ptr_if_codegen contains a CHECK-NOT
directive using an undefined DECL FileCheck variable. It seems copied
from target_data_use_device_ptr_codegen where there's a CHECK for a load
that defined the variable. Since there is no corresponding load in this
testcase, the simplest is to simply forbid any store and get rid of the
variable altogether.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D99771