Drop btrfs support

The btrf-progs package was found to contain GPL-3 code (see APERTIS-6710), so the images no longer use btrfs filesystems. Ribchester was made filesystem agnostic in June 2017 so let's get rid of the old, deprecated Btrfs-specific code.

Tested on the apertis_v2021dev1-target-amd64-uefi_20200206.0 image, once installed and having rebooted the HMI comes up and the Songs application displays the album art.

The ribchester testcase passes as well when running the steps manually (and adjusting to use the locally build ribchester packages).