rcutorture: remove synchronize_rcu from readers

rcutorture: remove synchronize_rcu from readers

This gives much worse numbers for readers, especially if synchronize_rcu
is made more expensive as is the case with --enable-membarrier.  Before:

   $ tests/rcutorture 10 stress 10
   n_reads: 98304  n_updates: 529  n_mberror: 0
   rcu_stress_count: 98302 2 0 0 0 0 0 0 0 0 0

After:

   $ tests/rcutorture 10 stress 10
   n_reads: 165158482  n_updates: 429  n_mberror: 0
   rcu_stress_count: 165154364 4118 0 0 0 0 0 0 0 0 0

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>