ir_to_mesa: Remove pretense of support for function calls.

ir_to_mesa: Remove pretense of support for function calls.

While ir_to_mesa contains code that attempts to support functions, I
honestly doubt it's been tested and have little confidence that it
works.

The comment in visit(ir_function *ir) doesn't inspire confidence:
/* Ignore function bodies other than main() -- we shouldn't see calls to
 * them since they should all be inlined before we get to ir_to_mesa.
 */

Furthermore, hardware drivers such as i915, i965, and (AFAICT) r200
don't support the BGNSUB/ENDSUB/CAL opcodes anyway.  Only swrast does.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>