i965: Remove the brw_vue_prog_key base class.

i965: Remove the brw_vue_prog_key base class.

The legacy userclip fields are only used for the vertex shader, and at
that point there's only program_string_id and the tex struct, which are
common to all keys.  So there's no need for a "VUE" key base class.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>