glsl: Clean up case statement in builtin_variables.cpp's add_variable.

glsl: Clean up case statement in builtin_variables.cpp's add_variable.

The case statement purported to handle the addition of ir_var_const_in
and ir_var_inout builtin variables.  But no such variables exist.
This patch removes the unnecessary cases, and adds a comment
explaining why they're not needed.

Reviewed-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>