i965: Remove useless IR self-destruct backend_shader method.

i965: Remove useless IR self-destruct backend_shader method.

From the point it's constructed the CFG contains the only existing
copy of the program IR, and it never becomes invalid.  Calling
backend_shader::invalidate_cfg would have destroyed the program
structure irrecoverably -- We weren't calling it at all for a good
reason.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org
Reviewed-by: Matt Turner <mattst88@gmail.com>