This is not user-friendly. Imagine wanting to add 50 legend items a couple of times to check for improvements and having to wait each time that they'd go away "eventually."
Not only do I not get an option for bulk importing my perfectly named icons for a feature layer, but also, I hit the "Maximum number of symbols reached." and have to wait for them to decide when I can continue. Just Ridiculous. Bad practices are prevalent in ESRI everywhere. Very lousy programming in the UI of the final products for designers and developers.
I was just doing some testing and getting annoyed with the multiple versions of symbols I had uploaded, was looking for how to remove them and found myself here.
I'm going to follow up to the "they will disappear eventually" solution (in case others follow the same path of googling that I did). It seems these are stored in cookies, so clearing the right cookie (or all associated with arcgis.com) did clear that list of symbols out for me.