Removed FolderFeedRemove class. Instead the action of removing a feed from its current folder is performed with  a Feed instance method.

This means less code duplication. Before, the code to remove a feed from its current folder, for a given user, was duplicated in:

- the Feed callback that run when a feed was added to a folder (so that the feed was removed from its old folder before being added to a new one)
- the FolderFeedRemove class, which was invoked when a user explicitly removed a feed from its folder by clicking on "None" in the folder dropdown menu.