intel#4020 introduced some kind of wrapper for SYCL kernels on host
to be able to launch them on ESIMD emulator backend. Even though we had
dropped that feature since then, we didn't remove corresponding
`handler.hpp` modifications. This PR removes them.

The main effect expected from this PR is compilation time improvement:
those kernel wrappers are specialized by kernel name, meaning that there
will be plenty of extra useless functions emitted during host compilation
pass for every kernel in a program.

This patch also uncovered a missing case in
`HostKernel::InstantiateKernelOnHost` which we couldn't ever encounter
because we transformed a host kernel to always accept `nd_item`, thus
always skipping problematic `item` code path.

This PR is not expected to introduce any functional changes, but since
I'm not very familiar with the SYCL RT, I'm not entirely sure of that.

…-esimd-emulator-legacy