MINOR: [R] Remove outdated TODO for integer division by zero tests

[HyukjinKwon]

Rationale for this change

63b8b7e: ARROW-13800 Added the TODOs
2468992: ARROW-14297 Added the tests but forgot to remove TODOs.

arrow/r/tests/testthat/test-dplyr-funcs-math.R

Lines 455 to 456 in 744f0ec

	int_div_zero_int = integers %/% 0L,
	int_div_zero_dbl = integers %/% 0,

arrow/r/tests/testthat/test-dplyr-funcs-math.R

Lines 459 to 460 in 744f0ec

	dbl_div_zero_int = doubles %/% 0L,
	dbl_div_zero_dbl = doubles %/% 0

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.

[jonkeane]

Thanks for this! One small comment, I don't mind duplicating the test behavior in the place where the todo was so that we have all the arithmetic tests in one place (even though they are also tested in the dplyr tests too, we already have a bunch of overlap there too)

[jonkeane]

I wouldn't mind also having a test here as well as the dplyr tests. Or alternatively: maybe we mention that the %/% tests are in the dplyr math file?