Update notes about IRC channels

Hi. Not sure where the Krusader crew has been left after the implosion of Freenode, but after checking krusader.org which has no mention of any IRC channels and #krusader being an empty room on Libera Chat, I assume you do not currently have resources to support users/developers on IRC, thus proposing to simply remove the mention completely from docs and metadata :)

Also separate commit to update the link to #kde-devel.