Unneded filesystem sandbox holes for home and music

The permissions
```
    "--filesystem=xdg-download:ro",
    "--filesystem=xdg-music:ro",
```
are not needed, the app uses the file chooser portal which grants access to whatever file in the host as long as the DE supports the portal.

I see https://github.com/vixalien/decibels/issues/24 and https://github.com/vixalien/decibels/issues/25 were fixed by this, but I am not convinced.