Revert "Feat: enrollments can expire (#1989)"

[angela-tran]

This reverts commit 479b249d6059a7d7148aef8bdb5d5fc3066d6d17, reversing changes made to 96ea690df50b556cf8ee15bff606cfb2e59cf3e9.

We're reverting the change to how enrollment is performed to unblock other in-progress changes that we'd like to be able to deploy.

This reverts all the logic for looking up funding sources linked to a group, and determining if a user is eligible for re-enrollment. Instead we go back to handling the 409 error that indicates a user is already enrolled in a group, with no logic for expiration.

We are working with Littlepay to identify issues with testing this expiration logic, and will bring these changes back to dev once we solve those issues.

[github-actions[bot]]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
benefits
settings.py
benefits/enrollment
views.py
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