Curious as to why you would want to clear it?
This is an internal function and not exposed via the external API. However, you could get it working via a somewhat roundabout way by calling a node script similar to this one - Command to regenerate mpd.conf - #8 by ashthespy.
You would need to adjust the endPoint keys to "endpoint" : "miscellanea/albumart", "method" : "clearAlbumartCache"
HTH