rcutorture: Remove obsolete TREE08-T.boot file

The TREE08-T.boot file was used to provide alternative options for
debugging, but things have changed, it has not kept up, and it has not
been used or missed.  This commit therefore removes it.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/configs/rcu/TREE08-T.boot

-rcutree.rcu_fanout_exact=1