Clean up remains of field_pos_list

[lpenz]

The field is no longer used anywhere, but it was still in a function doc and as an argument for Padding.self_build.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.10%. Comparing base (8f9c7a6) to head (9511cb9).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4714      +/-   ##
==========================================
- Coverage   82.11%   82.10%   -0.02%     
==========================================
  Files         361      361              
  Lines       86908    86908              
==========================================
- Hits        71365    71356       -9     
- Misses      15543    15552       +9     

Files with missing lines	Coverage Δ
scapy/packet.py	84.48% <100.00%> (ø)

... and 10 files with indirect coverage changes

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[gpotter2]

Thanks for the PR & sorry for the delay.