I don't think this is a notification. This is the recent apps list."Fixable"? Probably not, it's designed that way. If you swipe it left, you should get the choice of never seeing it again or minimizing it, but that's about all.
There are two kinds of notifications, "clearable" and "not clearable". That one (there are others) are the not clearable type.
Is it just me or is the calendar pinned? If it's pinned, it wouldn't clear. Click the pin icon on the top to unpin... It should clear.