[OpenMP] Fix libomptarget build issue (llvm#74067)

Libomptarget cannot be build because of the recent refactoring
introduced in patch 148dec9 :
[OpenMP][NFC] Separate Envar (environment variable) handling (llvm#73994)

That patch moved handling of environment variables from libomptarget
library. That's why we don't need usage of "llvm::omp::target" namespace
if we handle environment variables.

Author: DominikAdamski

openmp/libomptarget/src/interface.cpp

@@ -454,7 +454,6 @@ EXTERN void __tgt_target_nowait_query(void **AsyncHandle) {
   // for the device operations (work/spin wait on them) or block until they are
   // completed (use device side blocking mechanism). This allows the runtime to
   // adapt itself when there are a lot of long-running target regions in-flight.
-  using namespace llvm::omp::target;
   static thread_local utils::ExponentialBackoff QueryCounter(
       Int64Envar("OMPTARGET_QUERY_COUNT_MAX", 10),
       Int64Envar("OMPTARGET_QUERY_COUNT_THRESHOLD", 5),