intel/vec4: Remove Gen8+ support from vec4 backend

After some discussion in !5236, it seems it's time to remove the vec4 support for Gen8+. There are still some references to Gen8+ in brw_vec4_gs_visitor.cpp and brw_vec4_tcs.cpp. These are in common code that is used for scalar compiles to those shader targets. Those functions could probably be moved to... some file that doesn't have vec4 in the name?