Remove unused unimplemented error due to lint

A quick check shows that this is not used anywhere else in the codebase.

        $ ag unimplemented
        backend/player/dlna/dlnaplayer.go
        34:var unimplemented = errors.New("unimplemented")

backend/player/dlna/dlnaplayer.go

@@ -31,8 +31,6 @@ const (
 	paused  = 2
 )
 
-var unimplemented = errors.New("unimplemented")
-
 type proxyMapEntry struct {
 	key string
 	url string