update_deps: make --clean-repo switch work in all cases
update_deps: make --clean-repo switch work in all cases

--clean-repo right now will raise an exception if the repository
directory was not present.  This makes it work in all cases.

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