NDEBUG is part of the C standard and disables asserts. Only this will
now be used to decide if asserts are enabled.
DEBUG was a Blender specific define, that has now been removed.
_DEBUG is a Visual Studio define for builds in Debug configuration.
Blender defines this for all platforms. This is still used in a few
places in the draw code, and in external libraries Bullet and Mantaflow.
While it may be useful to detect threading issues for low poly meshes in debug
builds, it can also cause confusion when one breaks but not the other.
Further, having this logic for just a handlful bmesh functions while everything
else does not makes little sense.
