Fix slime-all-xrefs

Commit 6779941dc364666fef3b705ed8cf8bf97022d4b9 changed the behaviour of `slime-next-line/not-add-newlines` so that it returns 1 at end of buffer instead of erroring, which broke `slime-all-xrefs`.

`slime-next-line/not-add-newlines` seemed otherwise unused, so deleted that and just used `forward-line` directly.