Remove the setting jupyter.enableKeyboardShortcuts

[DonJayamanne]

As we have a keymap extension this is no longer relevant

[joyceerhl]

I put in a deprecationMessage for this setting. I think a deprecation message is better than deleting the settings immediately because it is an opportunity to educate users on what to migrate to instead. I would suggest that we only delete the setting 1-2 releases from now.

[greazer]

Perhaps we use the opportunity here to remove all other deprecated settings. Need to check telemetry to make sure we don't pull something that we shouldn't.

[greazer]

Dupe of #7094