glsl: remove unecessary flags.q.subroutine_def

This bit is definitely not necessary because subroutine_list
can be used instead. This frees one more bit in the flags.q
struct which is nice because arb_bindless_texture will need
4 bits for the new layout qualifiers.

No piglit regressions found (including compiler tests) with
"-t subroutine".

v2: set the subroutine flag for validating illegal flags

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/glsl_parser_extras.cpp

@@ -1075,7 +1075,7 @@ _mesa_ast_type_qualifier_print(const struct ast_type_qualifier *q)
    if (q->flags.q.subroutine)
       printf("subroutine ");
 
-   if (q->flags.q.subroutine_def) {
+   if (q->subroutine_list) {
       printf("subroutine (");
       q->subroutine_list->print();
       printf(")");