[SYCL][CUDA][HIP] Remove XFAIL from tests failing due to usm_prefetch

[AidanBeltonS]

CUDA and HIP previously failed due to a placeholder flag being set.
This has been removed as the flag should not change the functions behaviour.
This patch removes the XFAIL from USM/prefetch.cpp and USM/dep_events.cpp as they
should now be expected to pass for CUDA and HIP.

Text from llvm PR #4568

Patch removes assertions in pi_cuda, pi_hip, and pi_level_zero which fail based upon the flag given to the function.
prefetch_usm function now does not pass flag to PI.
The flag is a placeholder and should be ignored.

This is a follow up PR from issue #4467