intel/vec4: Remove all support for Gen8+ [v2]

intel/vec4: Remove all support for Gen8+ [v2]

v2: Restore the gen == 10 hunk in brw_compile_vs (around line 2940).
This function is also used for scalar VS compiles.  Squash in:

    intel/vec4: Reindent after removing Gen8+ support
    intel/vec4: Silence unused parameter warning in try_immediate_source

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> [v1]
Reviewed-by: Matt Turner <mattst88@gmail.com> [v1]
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> [v1]
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6826>