Remove CONF.volume_feature_enabled.bootable

The option is deprecated already and the os-set_bootable feature has
been implemented since Ib6cdca15950fe86b4fb2a43cfc7338e28260e453 .
That was 2.5 years ago. So we can remove this option.

Change-Id: I83085bc9e9747a5f427d1651408b265cc1e2030e
3 files changed