fix: when deleting the enrollment remove default private and self encryption keys.

[sitaram-kalluri]

- What I did
Fixes the issue: #2118

• When an enrollment is deleted, removes the default encryption private key and default self encryption key which becomes stale and are of no use.

- How I did it

• In the enroll_verb_handler.dart, after deleting the enrollment id, delete the "apkam public key", "default encryption private key", default self encryption key" from the keystore.

- How to verify it

• Updated the existing functional test to verify the changes.

- Description for the changelog

• fix: when deleting the enrollment remove default private and self encryption keys.