scripts: Mod spec.py to use local validusage.json
scripts: Mod spec.py to use local validusage.json

This script by default used the latest version of validusage.json from
the web. Changed to use the local copy checked into the repo. Used the
json-file option to fetch from a non-default location, removed the
-json arg and the URL option entirely.

Change-Id: Id088b08c8e66a3cdfca2b9e771797bffe941e5d3

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers