Replace manual USM aspect selector by sycl::aspect_selector

[keryell]

Several exercises are still doing manual work by defining their own USM aspect selector while in SYCL 2020 there directly the concept of sycl::aspect_accessor.
https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:device-selector