layers: Clean up core_validation GPA def & handling
layers: Clean up core_validation GPA def & handling

Got rid of artificially separations in calling GetProcAddr, remove
ext enable-checking at GPA-time -- this happens in CV.

Change-Id: I89815c86d7f9187537efd3568edd11a9ffffa44f

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools