(PC-33683) feat(NC): remove getExchangeRates from firestore

[lbeneston-pass]

Link to JIRA ticket: https://passculture.atlassian.net/browse/PC-33683

Flakiness

If I had to re-run tests in the CI due to flakiness, I add the incident on Notion

Checklist

I have:

• Made sure my feature is working on web.
• Made sure my feature is working on mobile (depending on relevance : real or virtual devices)
• Written unit tests native (and web when implementation is different) for my feature.
• Added a screenshot for UI tickets or deleted the screenshot section if no UI change
• If my PR is a bugfix, I add the link of the "résolution de problème sur le bug" on Notion
• I am aware of all the best practices and respected them.

Screenshots

delete if no UI change

Platform	Mockup/Before	After
iOS
Android
Phone - Chrome
Desktop - Chrome

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud