[emacs/evil] unbind C-k (instead use C-S-k) and down-mouse-1

[emacs/evil] unbind C-k (instead use C-S-k) and down-mouse-1

- i prefer C-k to be kill-line and instead use C-S-k for
  evil-insert-digraph

- unbind down-mouse-1 in evil-motion-state-map, which causes evil to
  enter visual mode when clicking a link with mouse, for instance in a
  help buffer