remove trivial goals from sessions

examples/WP_revisited/wp2/why3session.xml

examples/WP_revisited/wp2/why3shapes.gz

examples/avl/avl/why3session.xml

examples/avl/avl/why3shapes.gz

examples/avl/priority_queue/why3session.xml

examples/avl/priority_queue/why3shapes.gz

examples/avl/ral/why3session.xml

examples/avl/ral/why3shapes.gz

examples/avl/tables/why3session.xml

examples/avl/tables/why3shapes.gz

examples/bag/why3session.xml

examples/bag/why3shapes.gz

examples/bellman_ford/why3session.xml

examples/bellman_ford/why3shapes.gz

examples/bignum/why3session.xml

examples/bignum/why3shapes.gz

examples/binomial_heap/why3session.xml

examples/binomial_heap/why3shapes.gz

examples/bitwalker/why3session.xml

examples/bitwalker/why3shapes.gz