irqdesc: delete now orphaned references to timer_rand_state

In commit c5857ccf293 ("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: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>