st/mesa: drop the lowering of quad strips to triangle strips

st/mesa: drop the lowering of quad strips to triangle strips

This fallback to triangle strips is silly and should be done in drivers
if they need it.

This should fix the case when quad strips are used with flatshading that is
enabled by the "flat" GLSL varying modifier. It also fixes primitive restart
for quad strips.

This fixes piglit:
  NV_primitive_restart/primitive-restart-draw-mode-quad_strip

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Brian Paul <brianp@vmware.com>