Bug 1087200 - [Ringtones] close the pick activity window when it is in the background

This line is unnecessary, since `tonePlayer` stops itself when hidden: https://github.com/mozilla-b2g/gaia/blob/2824b499d39186ded447d21fc6b10302026d6e9d/apps/ringtones/js/tone_player.js#L39-L44