GH-50236: Remove obsolete OpenSUSE 15.5 workarounds

[Shally-Katariya]

Rationale for this change

Apache Arrow no longer supports the OpenSUSE 15.5 build configuration that required explicit shared_ptr and unique_ptr constructions.

This PR removes obsolete OpenSUSE 15.5-specific workarounds and simplifies the affected return statements by relying on standard implicit conversions.

What changes are included in this PR?

• Removed OpenSUSE 15.5-specific workaround comments.
• Replaced explicit std::shared_ptr<T>(std::move(...)) constructions with direct returns.
• Replaced explicit std::unique_ptr<T>(std::move(...)) constructions with direct returns.
• Simplified code in the affected Arrow C++ source files without changing functionality.

Are these changes tested?

Yes.

The changes were verified by building the affected components and running the relevant compute test suite:

ninja arrow-compute-tests
ctest -R "arrow-compute" --output-on-failure

All tests passed successfully.

Are there any user-facing changes?

No. This change is an internal code cleanup and does not affect public APIs, behavior, or user-facing functionality.

• GitHub Issue: [C++] Drop outdated workarounds for OpenSUSE 15.5. #50236

[pitrou]

@Shally-Katariya Can you fix the C++ formatting? You should be able to do it using pre-commit run -a cpp