Revert "Add '_' prefix to variable not to be propagated to ITK project"

This reverts commit 3ef5017689f49adb5943359b7360c58036e0cd6b.
Merge topic 'SIMPLEITK-472_Enable64BitsIDSOnWindows' into next

3ba71950 Revert "Add '_' prefix to variable not to be propagated to ITK project"