Our usage of the Youtube Data API v3 does not seem to require oauth credentials. Thus we can remove the CLIENT_ID variable and just use a simple API key, eliminating one more variable we have to set.
Update instructions for how to get youtube api key.
See: https://github.com/dasl-/piwall2/commit/4721136eaf41408479a48b063cab3fce36f7dfac
https://github.com/dasl-/piwall2/commit/5eb99a1bc4fd6abb9d162ade3a87d828a9a31ce4