[TIMOB-17484] iTunesArtwork files should not be packaged with app per Apple recommendations

The ti build command should remove the iTunesArtwork(@2x) files prior to packaging when the target is dist-appstore as is advised by Apple: https://developer.apple.com/library/iOs/qa/qa1686/_index.html These files are used only for iTunes (desktop app) syncing with AdHoc builds or during app development. The high-res icon developers upload to the iTunes portal as part of publishing is used by the iTunes sync with apps published to the app store. Removing these files from the bundle could make for measurably smaller files.