MINOR: [R] Remove outdated TODO for integer division by zero tests (#48653)

### Rationale for this change

63b8b7eb22: ARROW-13800 Added the TODOs
24689928da: ARROW-14297 Added the tests but forgot to remove TODOs.


https://github.com/apache/arrow/blob/24689928da8b122df89f29b63cffa75845058d4e/r/tests/testthat/test-compute-arith.R#L67


https://github.com/apache/arrow/blob/24689928da8b122df89f29b63cffa75845058d4e/r/tests/testthat/test-compute-arith.R#L70


https://github.com/apache/arrow/blob/24689928da8b122df89f29b63cffa75845058d4e/r/tests/testthat/test-compute-arith.R#L79-L80

### What changes are included in this PR?

This PR proposes to remove the obsolete todo about integer division by
zero tests.

### Are these changes tested?

No, I did not test this.

### Are there any user-facing changes?

No, dev-only.