Remove asdf-unit-schemas from dependencies

[sunpoet]

Description

The schemas provided by asdf-unit-schemas were duplicates of the schemas provided by asdf-standard.

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[braingram]

Thanks for the PR!

The change to pyproject.toml looks good. However there are other references to asdf-unit-schemas in the CI (and other files). This reminded me I had a draft PR with most of the changes. I cleaned that up and brought it out of draft:
#1767
I'm closing this PR in favor of the above one that removes other references to asdf-unit-schemas.