Remove the Yahoo protocol

Yahoo discontinued the Yahoo Messenger on July 17, 2018, 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
- remove the kconf_update scripts
- slightly update the documentation to not mention Yahoo anymore
- stop mentioning it in the appdata file
This way, developers/downstreams will not try to accidentally build/ship
this anymore.

BUG: 395287
Fixed-In: 18.08.0

[1] https://help.yahoo.com/kb/SLN28776.html