fix: remove Enzyme error hint for runtime activity

[gdalle]

The missing hint was spotted in #851. It is due to the combination of:

• Define error hints for abstract exception types JuliaLang/julia#58367 (error hints only work for concrete types)
• Fixup memory instance EnzymeAD/Enzyme.jl#2555 (EnzymeRuntimeActivityError is now a UnionAll type)

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.22%. Comparing base (a5fb081) to head (0ad1934).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...face/ext/DifferentiationInterfaceEnzymeExt/init.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #852      +/-   ##
==========================================
- Coverage   98.12%   97.22%   -0.90%     
==========================================
  Files         134      131       -3     
  Lines        7988     7964      -24     
==========================================
- Hits         7838     7743      -95     
- Misses        150      221      +71     

Flag	Coverage Δ
DI	97.68% <50.00%> (-1.13%)	⬇️
DIT	96.07% <ø> (-0.33%)	⬇️

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