Simplify MusiXmatch backend and remove unused code
Simplify MusiXmatch backend and remove unused code

ibmibmibm/beets,ibmibmibm/beets,ibmibmibm/beets,shamangeorge/beets,beetbox/beets,sampsyo/beets,shamangeorge/beets,beetbox/beets,sampsyo/beets,sampsyo/beets,beetbox/beets,shamangeorge/beets,shamangeorge/beets,sampsyo/beets,ibmibmibm/beets,beetbox/beets