irqdesc: delete now orphaned references to timer_rand_state

In commit c5857ccf29 ("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>

include/linux/irqdesc.h

@@ -10,12 +10,10 @@
 
 struct irq_affinity_notify;
 struct proc_dir_entry;
-struct timer_rand_state;
 struct module;
 /**
  * struct irq_desc - interrupt descriptor
  * @irq_data:		per irq and chip data passed down to chip functions
- * @timer_rand_state:	pointer to timer rand state struct
  * @kstat_irqs:		irq stats per cpu
  * @handle_irq:		highlevel irq-events handler
  * @preflow_handler:	handler called before the flow handler (currently used by sparc)