mbcollection: Fix default 'remove' value.
mbcollection: Fix default 'remove' value.

If the default is not None the value cannot be overriden by the user's
config.yaml.

beetbox/beets,jackwilsdon/beets,sampsyo/beets,SusannaMaria/beets,sampsyo/beets,SusannaMaria/beets,ibmibmibm/beets,sampsyo/beets,jackwilsdon/beets,ibmibmibm/beets,shamangeorge/beets,beetbox/beets,sampsyo/beets,ibmibmibm/beets,beetbox/beets,beetbox/beets,jackwilsdon/beets,shamangeorge/beets,ibmibmibm/beets,SusannaMaria/beets,SusannaMaria/beets,shamangeorge/beets,jackwilsdon/beets,shamangeorge/beets