Remove the WLM protocol

Summary:
Microsoft discontinued the Windows Live Messenger service in 2013, so
this protocol is effectively dead.

As a result:
- remove the code of the protocol itself
- remove all the supporting cmake stuff
- remove one notification specific to it
- slightly update the documentation to not mention WLM anymore
This way, developers/downstreams will not try to accidentally build/ship
this anymore.

Test Plan: Builds fine

Reviewers: kde-i18n-doc, pali, aacid

Reviewed By: aacid

Subscribers: kde-i18n-doc, #kopete

Tags: #kopete

Differential Revision: https://phabricator.kde.org/D12530