Clearing temporary assets from craft/storage/runtime
In Craft 2 the only real solution is to delete all the entries from craft_assetfiles where the sourceId is set to null. 

The reason why there's no clear cache tool for these is because that might allow for deletion of Assets that were just uploaded by someone else who might be about to use them.

This is really far from a practical solution and we know that, but it's not easily improved in Craft 2.

Craft 3 will slightly improve things by adding a "Temporary uploads" Asset volume which will hold all of the current user's leftover Assets, allowing you to deal with them as you would with regular Assets.