Remove ability to use internal callbacks

[DominicOram]

Fixes #615

Whilst this goes a bit further than the original issue in that it removes functionality I think this is fine as it's functionality that hasn't been used in a while and so unsure if it actually even works. All it was doing was serving to confuse us more than anything else. If we want the functionality back we can reintroduce it at a later date.

Acceptance Criteria

• Tests still pass (it's telling how little we had to change the tests)
• All the code around using external callbacks is removed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.88%. Comparing base (b8fd466) to head (46dedf3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #776      +/-   ##
==========================================
- Coverage   86.99%   86.88%   -0.12%     
==========================================
  Files         101      100       -1     
  Lines        6960     6939      -21     
==========================================
- Hits         6055     6029      -26     
- Misses        905      910       +5     

Components	Coverage Δ
i24 SSX	72.84% <ø> (ø)
hyperion	96.31% <100.00%> (-0.23%)	⬇️
other	96.58% <ø> (ø)