Some strings refers to closed source add-ons

[AntoineTurmel]

In this string, the software ask the user to install a closed-source add-on for MTP/MSC devices :

https://github.com/nightingale-media-player/nightingale-l10n/blob/master/locales/en-US/songbird/songbird.properties#L1401

We should remove those references and maybe redirect the user to the built-in FolderSync feature.

[rsjtdrjgfuzkfg]

Agree. Even if we get MSC or MTP device support at some time, I assume having autoplay is a low-priority feature.

[AntoineTurmel]

do we remove those strings ? I think we can remove them, and move all removed strings in an unused file to keep track if they decide to put things open source.

[rsjtdrjgfuzkfg]

agree for the unused-strings-file, see issue #17