intel: Move unlit centroid workaround into the elk compiler

This was only needed on Sandybridge.  We can delete the brw code,
and replace the generic devinfo bit with a helper inside the elk
compiler itself.

Thanks to Iván Briano for noticing we still had dead brw code for this.

Reviewed-by: Ivan Briano <ivan.briano@intel.com>
Part-of: <mesa/mesa!33764>