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GH-45508: [CI][R] Remove Ubuntu version from sanitizer jobs (#45509)

### Rationale for this change

`ubuntu-r-sanitizer` and `ubuntu-r-valgrind` use `wch1/r-debug` as their base image.

So we can't control Ubuntu versions for them.

### What changes are included in this PR?

Remove Ubuntu version from their configurations.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #45508

Lead-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Co-authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>