fix: remove policy mapping file when empty

[donatello]

Description

On a policy detach operation, if there are no policies remaining attached to the user/group, remove the policy mapping file, instead of leaving a file containing an empty list of policies.

Motivation and Context

With LDAP configured, after detaching all policies on a user, a user listing operation shows user with an empty policy list.

How to test this PR?

1. Configure LDAP for MinIO
2. Attach a policy to an LDAP user
3. Detach the policy from the LDAP user
4. List users with mc admin user list - it should not show the user from step 2/3.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[harshavardhana]

Please add a test-case

[klauspost]

LGTM - but no idea about the functionality.