snap/snapcraft.yml: rm ttf-ubuntu-font-family pkg

After upgrading the snap-pkg CI job from 20.04, it started
failing with:

```
Failed to fetch stage packages:
Error downloading packages for part 'gtk2':
The package ttf-ubuntu-font-family was not found..
```

We've tried alternatives for the package but they don't seem to
work; and just removing it makes it work again (and no apparent
issue in the GTK frontend).

snap/snapcraft.yaml

@@ -51,7 +51,6 @@ parts:
       - libgtk2.0-dev
     stage-packages:
       - libxkbcommon0  # XKB_CONFIG_ROOT
-      - ttf-ubuntu-font-family
       - dmz-cursor-theme
       - light-themes
       - adwaita-icon-theme