calendar: Only show 1 refresh button on calendar sources

Before this patch, both the "Update Calendar" and "Update calendar source" actions could be visible at once when right-clicking on a calendar source. The former is now hidden, as to avoid confusion. I believe it also did nothing at all - but am a bit unsure as I don't fully get the akonadi side of the code. Though, behavioural analysis (reading the logs of my Nextcloud server while clicking the button) didn't show anything happening when clicking it.