GHA/curl-for-win: drop libssh

GHA/curl-for-win: drop libssh

Switch back to default libssh2.

The distribution server has reliability issues (this time it works
locally though):
```
++ curl [...] --output pkg.bin https://www.libssh.org/files/0.11/libssh-0.11.3.tar.xz --output pkg.sig https://www.libssh.org/files/0.11/libssh-0.11.3.tar.xz.asc
curl: (92) HTTP/2 stream 1 was not closed cleanly: INTERNAL_ERROR (err 2) [4x]
```
Ref: https://github.com/curl/curl/actions/runs/18651134321/job/53169147048#step:3:2391

There is also no official mirror that I know of.

Ref: af8e1aa4b06e9dc78a559b485348e5464bd5cff5 #18257

Closes #19162