playlist: remove support for PLAYLIST_COMMAND_CLEAR
Although this breaks compatibility with playlist control
file versions 3-5, i.e. control files generated by any RB
build between Jan 29 2023 (commit 25bd3bc) and Oct 26 2024
(commit 7592d2c), compatibility with v2 of the control file
that was in use from 2003 to 2023 is maintained.
Change-Id: I7fd3394e31131bb0563e41d921bcaf940dd999bc