There should be a way to clear album art from files. Some users would want to strictly have separate cover art files in their music collection, as embedding images into music files repeatedly can take up space.
There are options in Picard to prevent from embedding images in the first place, but there is no way to remove cover art once it is already embedded in a file.
I think this could be easily implemented by adding another option when right clicking on the cover art preview. There is a difference between embedded and separate cover art, so the options could be "Remove cover art (embedded)" and "Remove cover art (file)" respectively, and they can each be greyed out depending on whether they exist or not.
I also feel that this would be a good niche feature, as other tag editors usually don't have this function.