Get rid of _mesa_vert_result_to_frag_attrib().

Get rid of _mesa_vert_result_to_frag_attrib().

Now that there is no difference between the enums that represent
vertex outputs and fragment inputs, there's no need for a conversion
function.  But we still need to be able to detect when a given vertex
output has no corresponding fragment input.  So it is replaced by a
new function, _mesa_varying_slot_in_fs(), which tells whether the
given varying slot exists as an FS input or not.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Brian Paul <brianp@vmware.com>