Considering that in VegaFEM v4.0, pthreads was replaced with
Intel TBB (see [1]), this commit removes the references to "pthread" and
also removes the corresponding custom CMake module.

[1] http://barbic.usc.edu/vega/download.html

It also fixes error like the following reported when integrating iMSTK in
project (e.g Slicer-based app) already looking up threading libraries
using "FindThreads" module provided by CMake:

   -- Found PThreads: C:/path/to/SlicerCustomApp-build/PThreads-install/include/pthread
    CMake Error at C:/path/to/SlicerCustomApp-build/iMSTK/CMake/Utilities/imstkFind.cmake:214 (add_library):
      add_library cannot create imported target "Threads::Threads" because
      another target with the same name already exists.
    Call Stack (most recent call first):
      C:/path/to/SlicerCustomApp-build/iMSTK/CMake/FindPThreads.cmake:8 (imstk_find_package)
      C:/path/to/SlicerCustomApp-build/VegaFEM-install/lib/cmake/VegaFEM/VegaFEMConfig.cmake:8 (find_package)
      C:/path/to/SlicerCustomApp-build/CTK/CMake/CMakeFindDependencyMacro.cmake:90 (find_package)
      C:/path/to/SlicerCustomApp-build/imstk-build/iMSTKConfig.cmake:144 (find_dependency)
      C:/path/to/SlicerCustomApp/Modules/CLI/MyCLI/CMakeLists.txt:33 (find_package)

List of VegaFEM changes:

$ git shortlog be55ca42b..b10672cf1 --no-merges
Jean-Christophe Fillion-Robin (1):
      COMP: Remove obsolete and unused PThreads requirement