Remove tsutil decapitate

[jeromekelleher]

#2269

Stacked on #2240

[codecov]

Codecov Report

Merging #2272 (e618415) into main (d69f956) will decrease coverage by 0.03%.
The diff coverage is 82.79%.

@@            Coverage Diff             @@
##             main    #2272      +/-   ##
==========================================
- Coverage   93.31%   93.27%   -0.04%     
==========================================
  Files          27       27              
  Lines       26040    26133      +93     
  Branches     1171     1172       +1     
==========================================
+ Hits        24299    24376      +77     
- Misses       1711     1727      +16     
  Partials       30       30              

Flag	Coverage Δ
c-tests	92.15% <78.26%> (-0.08%)	⬇️
lwt-tests	89.05% <ø> (ø)
python-c-tests	71.93% <66.66%> (-0.01%)	⬇️
python-tests	98.88% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
c/tskit/tables.c	90.17% <78.26%> (-0.12%)	⬇️
python/_tskitmodule.c	90.97% <93.33%> (+<0.01%)	⬆️
python/tskit/tables.py	98.89% <100.00%> (+<0.01%)	⬆️
python/tskit/trees.py	98.03% <100.00%> (+<0.01%)	⬆️

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[jeromekelleher]

This is after getting pulled in to #2240 so simplest to close