apacheGH-36573: [CI] Remove Travis CI related files and mentions (apa…

…che#36741)

### Rationale for this change

Because we can't use Travis CI in apache/arrow since 2023-01.

### What changes are included in this PR?

Remove Travis CI related files and mentions. Thanks Travis CI so far!

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: apache#36573

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>