In commit c5857ccf ("random: remove rand_initialize_irq()")
the timer_rand_state was removed from struct irq_desc.  Hence
we can also remove the forward declaration of it and the kernel
doc information now too.

Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>