How to unmount and possibly delete old Ubuntu Core snap

When I type:

$ mount|grep snap
/var/lib/snapd/snaps/ubuntu-core_352.snap on /snap/ubuntu-core/352 type squashfs (ro,relatime)
/var/lib/snapd/snaps/ubuntu-core_423.snap on /snap/ubuntu-core/423 type squashfs (ro,relatime)
/var/lib/snapd/snaps/ubuntu-core_122.snap on /snap/ubuntu-core/122 type squashfs (ro,relatime)

I see several old snap "mounted" and available while only the latest version is used as current in /snap/ubuntu-core

It's not a big deal but do I need older ubuntu core ? Isn't this a bug there are not deleted ? Could I simply unmount and delete the files ?

  sudo snap remove core --revision 122
  sudo snap remove core --revision 352

So, basically passing the revisions to remove in the command line.

To check the ones you've installed you have to use snap list [snap-name] --all