chore: remove unused getOldFluidData

Related to #000

What does this MR do and why?

Unused function

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MR acceptance checklist

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Quality

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Quality checklist

• For the code that this change impacts, I believe that the automated tests validate functionality that is highly important to users. If the existing automated tests do not cover this functionality, I have added the necessary additional tests or I have added an issue to describe the automation testing gap and linked it to this MR.
• I have made sure that the sonar quality coverage is up to standards.
• I have considered the impact of this change on the front-end, back-end, and database portions of the system where appropriate and applied.
• I have tested this MR in all supported browsers or determined that this testing is not needed.
• I have confirmed that this change is backwards compatible across updates (migrate up needs a migrate down), or I have decided that this does not apply.

Performance, reliability and availability

• I am confident that this MR does not harm performance, or I have asked a reviewer to help assess the performance impact.
• I have considered the scalability risk based on future predicted growth.

Documentation

• The MR is named after the desired squash commit to feed the changelog linked to the current milestone.
• I have added/updated documentation (also updated if the changes feature a deprecation) or I have decided that documentation changes are not needed for this MR.

Security

• I have confirmed that if this MR does not contains any sensitive informations hidden in the changes.

Deployment

• When featured on a self-data project release, I have made sure my app version in the manifest and package.json is incremented and any relative changes to the permissions are clearly written and transmitted to Cozy.