Switch to official vtk 7 available in hombrew/science

[bblacey]

vtk is available in homebrew-science but packaged with python bindings that use hard coded paths to the macOS python interpreter resulting in segfault exceptions. We host the formula and companion bottle built with —without-python. Once the vtk team eliminates their hard-coded path problem, we should be able to eliminate this custom formula to reduce maintenance.

The homebrew team made some progress here - https://github.com/Homebrew/homebrew-science/pull/4342 but including the python bindings still causes references to the system python interpreter to be included in the Salome dylibs. Additional context from the homebrew community https://github.com/Homebrew/homebrew-science/issues/3401

[bblacey]

It looks like there is progress by the vtk team that may make this possible. See:

Homebrew #3811 has been merged that includes VTK: fix linking to python

[bblacey]

The upstream fix still doesn't resolve the "segmentation fault" we get almost immediately upon launch when using hombrew/science/vtk that defaults to --with-python but purportedly fixed so there are no longer references to the system python...

 ./bin/FreeCAD --run-test 0
FreeCAD 0.17, Libs: 0.17R9686 (Git)
© Juergen Riegel, Werner Mayer, Yorik van Havre 2001-2016
  #####                 ####  ###   ####  
  #                    #      # #   #   # 
  #     ##  #### ####  #     #   #  #   # 
  ####  # # #  # #  #  #     #####  #   # 
  #     #   #### ####  #    #     # #   # 
  #     #   #    #     #    #     # #   #  ##  ##  ##
  #     #   #### ####   ### #     # ####   ##  ##  ##

*** Abort *** an exception was raised, but no catch was found.
	... The exception is:SIGSEGV 'segmentation violation' detected. Address 18

This will have to stay on the list a bit longer...

[bblacey]

The root cause for the crash was due to incorrectly configured dynamic loader paths that resulted in loading binaries external to the bundle. PR 535 resolves the path issues that prevented us from using the production VTK with python bindings.

[bblacey]

PR FreeCAD/FreeCAD#554 is also required and was just merged by Yorik. All green for moving to standard Homebrew VTK install.

[bblacey]

Re-generating the ports-cache to cache the Homebrew vtk port to fully resolve this issue.