gallium/util: remove the fake format helpers for bptc and etc2

gallium/util: remove the fake format helpers for bptc and etc2

This was a silly hack that kept growing and growing. Instead, just write
NULLs for those functions. No need to have helpers that just assert(0)
when you call them.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>