Revert the background job to clear unreferenced state groups (that was introduced in v1.126.0rc1), due to a suspected issue that causes increased disk usage. (#18222)

Revert "Add background job to clear unreferenced state groups (#18154)"

This mechanism is suspected of inserting large numbers of rows into
`state_groups_state`,
thus unreasonably increasing disk usage.

See: https://github.com/element-hq/synapse/issues/18217



This reverts commit 5121f921 (#18154).

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Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>