[CI][Rust] Remove duplicated nightly job

[asfimport]

We run the same test on each push with GitHub Actions.

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #5978

_{Note: This issue was originally created as ARROW-7333. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 5978
#5978