kiji-express-music: EXP-300. Remove references to KijiSlice from kiji-express-music.

issue: https://jira.kiji.org/browse/EXP-300
review: https://review.kiji.org/r/1366
commit: https://github.com/kijiproject/kiji-express-music/commit/0dadaec8d81dd0011f14d42bbde473405c0c4bf7