torture: Remove obsolete files from rcutorture .gitignore

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

tools/testing/selftests/rcutorture/.gitignore

 initrd
-linux-2.6
 b[0-9]*
-rcu-test-image
 res
 *.swp