BackupHelper doesn't clean up its signal subscriptions

The QObject::connect() call made in BackupHelper::set_storage_framework_socket() is never cleaned up.

The shared_ptr<QLocalSocket> passed in comes from unity::storage::qt::client::Uploader's socket() method. If storage-framework reuses that object -- which, in local_client/UploaderImpl.cpp, it does -- then we're making N concurrent signal connections to the same object.

This is probably the cause of the 'cumulative bytes of all tasks' issue that Xavi was seeing a week or so ago.