remove legacy config

[samfreund]

Description

removes the legacy config as it's no longer used. This breaks support for anyone who has settings saved from pre-2025.

Stalled until post champs

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2024.3.1
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added

[samfreund]

also #1488 is related

[samfreund]

I need to update a test, but as this is blocked until after champs, it's not a priority.