glsl: remove unecessary flags.q.subroutine_def

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>