Clean up dead code in illink targets (#85741)

Clean up dead code in illink targets (#85741)

Removes TFM version checks, and adds a warning when using
explicit ILLink package reference.

We reference ILLink as a packagereference that versions with the
runtime, so the 8.0 linker is only supported when targeting
`net8.0`, and we can remove the TFM checks. We don't want to
support someone adding a PackageReference to an 8.0 linker, while
targeting an older TFM, in an attempt to get newer linker bits
but with settings that were closer to what we shipped with that
TFM.

---------

Co-authored-by: Marek Safar <marek.safar@gmail.com>
Co-authored-by: Vitek Karas <10670590+vitek-karas@users.noreply.github.com>