GH-49133: [C++] Remove todo asking to use assign_by_moving

[HyukjinKwon]

Rationale for this change

The TODO comment in SmallVector's move assignment operator asks whether to use assign_by_moving() instead of destroy() + move_construct(). However, using destroy() + move_construct() is faster in most of the cases by memcpy according to the benchmark I ran:

Type	Elements	destroy + move_construct	assign_by_moving
uint8_t*	16	3.54 ns	8.71 ns
const uint8_t*	16	3.54 ns	8.71 ns
const void*	3	1.49 ns	5.00 ns
int	2	1.38 ns	5.52 ns
std::string_view	2	8.42 ns	3.78 ns
std::pair<int64_t, int64_t>	1	1.40 ns	2.16 ns
FakeArrowStruct	4	3.54 ns	6.32 ns

(I did not include the benchmark code I ran to make the pr description shorter. Can be provided upon request)

We could sophisticatedly benchmark, design and consider other cases but I believe this is not worthwhile.

What changes are included in this PR?

Removed the TODO comment.

Are these changes tested?

No I did not test.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Remove todo asking to use assign_by_moving #49133