Remove batched_migrations_parallel_execution flag

What does this MR do and why?

This MR batched_migrations_parallel_execution feature flag and some code that is not used anymore.

The flag was enabled for GitLab.com on 2023-01-25, and enabled by default in 15.11 - !114666 (merged).

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MR acceptance checklist

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Related to #372316 (closed)