Remove vestigal python install code from CMake build system

[ssteinbach]

• In the current build system, a correct python installation requires using setup.py, either through pip install or python setup.py install.
• there was code that was always attempting to install python headers, even in C++ only builds
• ...and python building was on by default, even though it was incorrect when only building through cmake.

[codecov-commenter]

Codecov Report

Merging #1113 (3f7c583) into master (0cc00ce) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1113   +/-   ##
=======================================
  Coverage   86.20%   86.20%           
=======================================
  Files         191      191           
  Lines       19028    19028           
  Branches     2105     2105           
=======================================
  Hits        16404    16404           
  Misses       2078     2078           
  Partials      546      546           

Flag	Coverage Δ
unittests	86.20% <ø> (ø)

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