[UI] Deleted the hint about several XMPP provider

Since Google, GMX, 1&1 are dropping XMPP support.

Change-Id: I6862e5f5510656842b9f6c4f311a484fd87fffd4
 
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/2023
Tested-by: Jenkins CI
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>