fix(07): fib_pos: show hypothesis, simplify proof
fix(07): fib_pos: show hypothesis, simplify proof

Show the hypothesis of the last case of `fib_pos` for consistency with the other cases and remove the last step of the proof. Having the hypothesis makes it more obvious that the last step is unnecessary.
avigad/tutorial,fpvandoorn/tutorial,Hodapp87/cs6460_project,leanprover/tutorial,soonhokong/lean-tutorial,Hodapp87/cs6460_project,leanprover/tutorial,avigad/tutorial,fpvandoorn/tutorial,fpvandoorn/tutorial,soonhokong/lean-tutorial,leanprover/tutorial,soonhokong/lean-tutorial,Hodapp87/cs6460_project,avigad/tutorial