Delete minus1Trunc.v, close #434

You can do something else, like `let is_trunc_hyp_by_typeclass_resolution := constr:(_ : IsTrunc n G) in ... <pass is_trunc_hyp_by_typeclass_resolution  to @Trunc_rect_nondep`.  Or `first [ (let test_goal_istrunc := constr:(_ : IsTrunc n G) in idtac) | fail 1 "Goal" G "is not necessarily" n "truncated" ]`.  But I think you should do something to ensure that the goal is n-truncated.