FIX-#7250: Revert "PERF-#6666: Avoid internal reset_index for left merge"

What do these changes do?

[x] first commit message and PR title follow format outlined here

NOTE: If you edit the PR title to match this format, you need to add another commit (even if it's empty) or amend your last commit for the CI job that checks the PR title to pick up the new PR title.
[x] passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
[x] passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
[x] signed commit with git commit -s
[x] Resolves #7250
[ ] tests added and passing
[x] module layout described at docs/development/architecture.rst is up-to-date