dlist: always use merged primitive for drawing

dlist: always use merged primitive for drawing

OpenGL 4.6 compatibility profile spec, Appendix B:

   21.  For any GL and framebuffer state, and for any group of GL commands and
        arguments, the resulting GL and framebuffer state is identical whether the
        GL commands and arguments are executed normally or from a display list.
        The only exception to this corollary is for built-in shader variables
        gl_VertexID and gl_PrimitiveID, which are not defined when drawing
        geometry within a display list.

(thanks Ian Romanick for pointing this out in piglit !419 MR)

Remove the code introduced in ebb228bec52a to determine if merged draws can be used.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11493>