Remove the documentation for the <s-x> keybinding

It was removed in commit 0e1632868e0377caf50db5d7e2361b7477533a9b and it
is now the standard kill-region command.
Remove the documentation for the <s-g> keybinding

It was removed in commit d49a8e9a16e2b3dacd55b4de3e526405cec6e13d and it
is now bound to the god-local-mode command.
Remove two keybinding tips concerning Super

- <s-f> was removed in commit d49a8e9a16e2b3dacd55b4de3e526405cec6e13d.
- <s-m> was removed in commit 0e1632868e0377caf50db5d7e2361b7477533a9b.
Merge pull request #642 from tow8ie/remove-old-keymap-documentation

Remove old keymap documentation