PERF-#7227: Call modin_frame.combine() for merge and join only when necessary

[anmyachev]

What do these changes do?

We should not call combine for cases where the calculations default to pandas, since the result of this function is not used.

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• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves Do right._modin_frame.combine() for merge and join only when necessary #7227
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date