Asset build file - remove unused vars and add defaults if build file can't be found

[bradp]

This needs some testing, I don't have the asset files locally and they don't seem to build, so this is my workaround for it.
It also removes the $version / $debug vars, as they aren't used.

[coreymcollins]

This all seems to work for me!

Steps to test:

1. Check out the branch
2. Run npm ci
3. Verify everything built properly
4. Test on the frontend to ensure stylesheets and scripts are loading as expected (they are!)
5. Manually delete index.asset.php file
6. Test on the frontend to ensure stylesheets and scripts are loading as expected (they still are!)
7. Run npm run build to regenerate the index.asset.php file
8. Test on the frontend to ensure stylesheets and scripts are loading as expected (they still still are!)

@bradp – I saw that you edited your initial comment with a strikethrough. Did anything change on your end that needs to be further tested?

[bradp]

Nope - the strikethrough was that I got my local fixed and able to build!

Also with this PR, we remove the issue of the theme fatal erroring if it isn't built, which is always a good thing to avoid those.