Revert "mesa: check for z=0 in _mesa_Vertex3dv()"

Revert "mesa: check for z=0 in _mesa_Vertex3dv()"

This reverts commit f04d7439a0ad6e13ff2912ff824553b6bcf511a4.

It no longer helps performance and the current vbo implementation is
faster anyway.

The app that hit this was a CAD program called Spazio3D. It made pretty
terrible use of the OpenGL API and we sent them some tips for improvements.
I'm assuming they've fixed this by now.

Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4052>