Mixin3D.shell: removed deprecated OCCT method

[marcus7070]

Will close #649.

I also shortened the code a bit, there was a few duplicated calls.

[codecov]

Codecov Report

Merging #829 (3192df9) into master (ab876ac) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 3192df9 differs from pull request most recent head e41b030. Consider uploading reports for the commit e41b030 to get more accurate results

@@           Coverage Diff           @@
##           master     #829   +/-   ##
=======================================
  Coverage   95.15%   95.15%           
=======================================
  Files          32       32           
  Lines        7489     7495    +6     
  Branches      798      799    +1     
=======================================
+ Hits         7126     7132    +6     
  Misses        224      224           
  Partials      139      139           

Impacted Files	Coverage Δ
cadquery/cq.py	91.42% <ø> (ø)
cadquery/occ_impl/shapes.py	92.47% <100.00%> (+<0.01%)	⬆️
tests/test_cadquery.py	99.20% <100.00%> (+<0.01%)	⬆️

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[jmwright]

I just had the one question. Other than that it looks good. Thanks @marcus7070