remove the key if there aren't any left callbacks when the remove function is called (#8)
remove the key if there aren't any left callbacks when the remove function is called (#8)

Co-authored-by: Georgi Davidkov <8335a01c9e4074fdcbba3d82b98b38a46b94d152@tick42.com>
kirilpopov/callback-registry,kirilpopov/callback-registry