Removes outdated docs of _extrude

[RubenRubens]

It's not a big deal.

[codecov]

Codecov Report

Merging #648 (f614780) into master (606eb27) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #648   +/-   ##
=======================================
  Coverage   94.03%   94.03%           
=======================================
  Files          31       31           
  Lines        6636     6636           
  Branches      726      726           
=======================================
  Hits         6240     6240           
  Misses        258      258           
  Partials      138      138           

Impacted Files	Coverage Δ
cadquery/cq.py	89.89% <ø> (ø)

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[jmwright]

Thanks @RubenRubens