MINOR: [R] Remove outdated TODO for integer division by zero tests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