test612: SCP `rm` the uploaded remote file (not the local source), unignore in CI

test612: SCP `rm` the uploaded remote file (not the local source), unignore in CI

It accidentally worked on all CI-tested operating systems, except on
native Windows.

Fixing:
```
=== Start of file stderr612
   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                  Dload  Upload   Total   Spent    Left  Speed
[...]
 curl: (21) rm command failed: Operation failed
```
Ref: https://github.com/curl/curl/actions/runs/14004029192/job/39215359241?pr=16781#step:15:1424

Also remove this test from the ignore list in GHA/windows.

Closes #16801