GH-47640: [CI] Remove needless ci/docker/ubuntu-22.04-csharp.dockerfile (#48298)

### Rationale for this change

Dockerfile is orphaned

### What changes are included in this PR?

Remove it

### Are these changes tested?

No

### Are there any user-facing changes?
No
* GitHub Issue: #47640

Authored-by: Nic Crane <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: thisisnic