fix key removal for re-shares

we need to check "file_source" instead of "item_source"

Steps to test:

- user1 creates folder which contains a file
- user1 shares the folder to user2
- user2 shares the file from the folder as public link
- check if all share-keys in data/user1/files_encryption/share-keys/folder exists (user1, user2 and pubkey)
- user2 removes the public link again
-  check if the pubkey was removed from the share-keys.