Saner scrolling behaviour in sly-mrepl

Also cleanup whitespace

* sly-mrepl.el (sly-mrepl-return, sly-mrepl--insert-prompt): Don't
use `sly-mrepl--recenter'.
(sly-mrepl--recenter): Removed.
(sly-mrepl-mode): Set three comint scrolling-related vars to nil.