cmake: Remove Vulkan::Registry

Vulkan::Registry behaves differently in `add_subdirectory` vs
`find_package` builds. Which can result in build failures.

Also Vulkan::Registry never made sense as a CMake target and
was very brittle in numerous ways.

Better to provide a `VULKAN_HEADERS_REGISTRY_DIRECTORY` to users
instead and remove Vulkan::Registry completely.

closes #351

README.md

@@ -20,6 +20,7 @@ Files in this repository originate from:
 
 * BUILD.gn
 * BUILD.md
+* cmake/
 * CMakeLists.txt
 * tests/*
 * CODE_OF_CONDUCT.md