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

BUILD.md

@@ -54,6 +54,8 @@ Which, when successful, will add library target called `Vulkan::Headers` which y
 find_package(VulkanHeaders REQUIRED CONFIG)
 
 target_link_libraries(foobar PRIVATE Vulkan::Headers)
+
+message(STATUS "Vulkan Headers Registry: ${VULKAN_HEADERS_REGISTRY_DIRECTORY}")
 ```
 
 ## Repository Set-Up