service: don't use the grandpa observer (#494)

* service: don't use the grandpa observer

* service: remove unnecessary boxing

* service: fix indentation

* service: remove unnecessary on_exit

tasks spawned with `spawn_task`/`spawn_essential_task` are already
guarded by `on_exit`.

* Update service/src/lib.rs

Co-Authored-By: Gavin Wood <[email protected]>