gallium: Drop PIPE_SHADER_CAP_PREFERRED_IR.

gallium: Drop PIPE_SHADER_CAP_PREFERRED_IR.

Now everyone's saying NIR, and doing any NTT internally.  The only returns
of TGSI were in gallivm_get_shader_param() and
tgsi_exec_get_shader_param(), but the drivers were returning NIR instead
of calling down to them.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23114>