Remove files from UploadBehavior::__filesToRemove after unlinking

This prevents the plugin from attempting to delete a file a second time
if an associated media file is deleted before the main model is saved.
Otherwise, the plugin will attempt to unlink the files in
UploadBehavior::__filesToRemove in UploadBehavior::afterDelete() and
then again in UploadBehavior::afterSave().
Merge pull request #175 from BallStateCBER/bugfix-redundant-delete

Remove files from UploadBehavior::__filesToRemove after unlinking