srcu: Remove dead code in srcu_gp_end()

Of course, compilers will optimize out a dead code. Anyway, remove
any dead code for better readibility.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>