Delete minus1Trunc.v, close #434

Oh, I see.  You could pull it out of the type of `T` within the `match`.  Perhaps a better alternative is to do `refine (@Trunc_rect_nondep _ _ _ _ _); [];` to ensure that we have only a single goal (no `IsTrunc` goal), or to stick a
```coq
match goal with
  | [ |- IsTrunc ?n ?G ] => fail 1 "Goal" G "is not necessarily" n "truncated"
  | _ => idtac
end
```
after the end of the `progress repeat match goal with`, to check that we've picked up all the goals by typeclass resolution.