Clean up libs\thunk\mock\mock.cpp _stop_service

### Describe the bug

https://github.com/microsoft/ebpf-for-windows/blob/main/libs/thunk/mock/mock.cpp#L185 says:
```
// TODO: (Issue# 852) Just a stub currently in order to compile.
// Will be replaced by a proper mock.
```

However issue #852 is closed as fixed. We could reopen that bug, but I decided to create a new one instead.

### OS information

_No response_

### Steps taken to reproduce bug

Code inspection.

### Expected behavior

Code should not reference a fixed bug as a TODO

### Actual outcome

Code references a fixed bug as a TODO

### Additional details

_No response_

Review libs/thunk/mock/mock.cpp around line 185, where _stop_service contains the TODO referencing issue #852. Confirm that issue #852 is closed, then update the stale comment so it no longer references a fixed issue. Done means the code no longer presents issue #852 as an active TODO; no specific test is mentioned.

Written by the indexing model from the issue text.