draw: nuke the interp parameter from vertex_info

draw: nuke the interp parameter from vertex_info

draw emit couldn't care less what the interpolation mode is...
This somehow looked like it would matter, all drivers more or less
dutifully filled that in correctly. But this is only used for emit,
if draw needs to know about interpolation mode (for clipping for instance)
it will get that information from the vs anyway.
softpipe actually used to depend on that interpolation parameter, as it
abused that structure quite a bit but no longer.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>