SQLAlchemy 2.0 follow-up

[jdavcs]

1. Remove cascade_backrefs=False:
   No longer needed after move to SQLAlchemy 2.0
   https://docs.sqlalchemy.org/en/20/changelog/migration_14.html#cascade-backrefs-behavior-deprecated-for-removal-in-2-0
2. Disable transaction helper.
   The helper checks if a transaction is active and begins one if not. There are 500+ instances where this helper is called. I'd like to first check that disabling it does not break transactional state; if this does not cause new session-related errors, I'll remove the function and the calls to it.
   Ref: Towards SQLAlchemy 2.0: drop session autocommit setting #15421

If this does not break transactional state, this helper should beremoved completely.

How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[nsoranzo]

Is the plan to remove the transaction helper as part of this PR if tests pass, or in a follow-up PR after some real-world testing?

[jdavcs]

Is the plan to remove the transaction helper as part of this PR if tests pass, or in a follow-up PR after some real-world testing?

@nsoranzo A follow-up PR, but very soon.