Silence failed jobs for horizon-rebase-fix-testing

As per Vivek, it is known that these jobs will continue failing for some more time,
till someone takes some time out and fixes them. Till that time, this commit is
silencing the IRC notification, which is unnecessarily flooding the channel.
Merge pull request #1 from JioCloud/quiesce-horizon-rebase-irc-notif

Silence failed jobs for horizon-rebase-fix-testing