Remove the DynamicPPL extension

[mhauru]

This extension is pirating functionality from DynamicPPL. Also, the piracy is now rendered moot by TuringLang/DynamicPPL.jl#1117.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.93%. Comparing base (e099b56) to head (498bac6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2748      +/-   ##
==========================================
- Coverage   69.93%   69.93%   -0.01%     
==========================================
  Files          58       57       -1     
  Lines       19323    19322       -1     
==========================================
- Hits        13513    13512       -1     
  Misses       5810     5810              

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