[offload][omp] Remove data_fence
All supported backends execute enqueued work on a given queue in
submission order (CUDA, AMDGPU, and Host queues are always in-order;
Level Zero's default and non-default in-order/synchronous command
modes are as well), so the explicit data-fence used to order a
pointer-attachment after prior data transfers is unnecessary. Remove
the DeviceTy/GenericDeviceTy/GenericPluginTy dataFence chain and the
now-unused olQueueBarrier liboffload API added to support it.
