COMP: Remove "-fopenmp" hack from External_ITKv4

This hack was first introduced while depending on ITK v4.0a07 (as
specified in Slicer r17596 comment). Following v4.0a07, ITK
build system has been updated to properly deal with the
issue (See kitware/ITK@8a135bcb).

Considering that
(1) Slicer now depends on a version of ITK including
commit kitware/ITK@8a135bcb
(2) variables "ITK_CMAKE_C_FLAGS" and "ITK_CMAKE_CXX_FLAGS" have
never been used to configure ITKv4 project
(3) variable "CMAKE_CXX_FLAGS" was improperly referenced as "CMAKE_CX_FLAGS"

the corresponding hack is removed from the external project.

See 0001739

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22710 3bd1e089-480b-0410-8dfb-8563597acbee