VULKAN_HEADERS_INSTALL was added since it was believed there was
a valid use case for it.
After looking into the use case that provoked this change there is
no reason to keep VULKAN_HEADERS_INSTALL as an option.
Here is the use case we do NOT want to support:
https://github.com/KhronosGroup/Vulkan-Headers/pull/416#issuecomment-1622318949
Fundamentally this problem is caused add_subdirectory/find_package
not being able to work together flawlessly.
Which isn't the responsibility of Vulkan-Headers to fix.
It's the responsibility of projects that consume Vulkan-Headers
to account for either method.
As described by the CMake maintainers:
https://discourse.cmake.org/t/idiomatic-way-to-handle-packages-and-add-subdirectory/8400
