BUG: Do not set CMAKE_CXX_STANDARD in ITKConfig

[blowekamp]

The UseITK file is the standard place where the global CMake variables were updated to the ITK configuration. Currently the use file calls ITKInitializeCXXStandard to initialize the CXX standard. Additionally, modern CMake is now using target properties for the CXX standard, making this approach not needed.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for
further development details if necessary.

[dzenanz]

Fine by me, but probably someone else should take a look at this small PR.