chore: remove all legacy related code (conversion functions and tests)

### What is this about?

MetaMetrics have evolved and preserved backward compatibility with legacy, making the code a bit complicated (so more bug risk).
Migrate all the legacy code to new and remove deprecated then unused legacy code: for instance the legacy trackEvent function and the legacy event types (but there's more)

### Scenario

_No response_

### Design

_No response_

### Technical Details

- remove or migrate code
- update unit tests
- update in code doc

### Threat Modelling Framework

_No response_

### Acceptance Criteria

- No remaining metametrics legacy code
- all test updated and pass

### Stakeholder review needed before the work gets merged

- [x] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

Start by locating the legacy MetaMetrics conversion functions, trackEvent implementation, and legacy event types, then trace their references in production code, unit tests, and documentation. Done means legacy code has been migrated or removed, no legacy references remain, and all updated tests pass.

Written by the indexing model from the issue text.