Commander should remove backup on uninstall

[fchavant]

Expected behavior

If a core instance has a backup (i.e because it has been upgraded) it should get deleted when that instance gets uninstalled.

Actual behavior

Backup stays on disk after the instance has been uninstalled.

Steps to reproduce

lisk core:install foo
lisk core:upgrade foo  # backup gets created as part of the upgrade process
lisk core:uninstall foo
ls -l ~/.lisk/backups/  # will show a backup of "foo"

Which version(s) does this affect? (Environment, OS, etc...)

2.2.0