Enhancement: Remove all snapshots using `forget --unsafe-allow-remove-all`

The forget command now supports the `--unsafe-allow-remove-all` option. It must
always be combined with a snapshot filter (by host, path or tag).
For example the command `forget --tag example --unsafe-allow-remove-all`,
removes all snapshots with tag `example`.

https://github.com/restic/restic/pull/4764