tag/Mask: fix yet another typo, this time in Unset()
Similar to commitse8f2f98048andff1ff1e54aCloses https://github.com/MusicPlayerDaemon/MPD/issues/783
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Similar to commitse8f2f98048andff1ff1e54aCloses https://github.com/MusicPlayerDaemon/MPD/issues/783