[C++] Drop outdated workarounds for OpenSUSE 15.5.

[pitrou]

Describe the enhancement requested

If you search for "opensuse" in the codebase, you can find lots of workarounds for C++ compiler limitations on OpenSUSE 15.5, such as:

arrow/cpp/src/arrow/compute/kernels/hash_aggregate_internal.h

Lines 58 to 59 in 1926639

	// R build with openSUSE155 requires an explicit unique_ptr construction
	return std::unique_ptr<KernelState>(std::move(impl));

However, we dropped our OpenSUSE build in #46990. So we should be able to remove those workarounds.

Component(s)

C++

[pitrou]

@kou Does this sound ok to you?

[kou]

Yes. Let's remove them.