Cherry pick commit from `main` for
#15581.

Reduces the `.librarian/state.yaml` and
`.librarian/generator-input/client-post-processing` content to what is
necessary for our initial client on the `preview` branch.

Discussed [internally
here](https://docs.google.com/document/d/1dEwVSpOlRkajh8ucHdBnwbkbrKIaHRbbsbEhjejARsE/edit?pli=1&tab=t.fg6dq8lbl6tj).

Ran local generation against `preview` branch of `googleapis`
successfully.

http://b/450631301

Removes all packages from `preview` branch, except for first API we will
test with (google-cloud-compute).

Sync the `packages/google-cloud-compute` from `main` to `preview`. When
`googleapis@preview` is getting regular updates, we will start using
`legacylibrarian` generate directly with a commit from
`googleapis@preview` and change to a preview version. For now, we just
want to make sure we aren't falling too far behind.

Executed with `git checkout main -- packages/google-cloud-compute
.librarian/generator-inpute/packages/google-cloud-compute` based on
`main` at
ea8ddeb.
Manually updated `.librarian/state.yaml` to match.

Regenerate `google-cloud-compute` on preview branch with latest commit
on `googleapis@preview`. There is no diff b.c its contents is the same
as the `googleapis@master` and I synced `google-cloud-python@preview`
branch with the latest from `google-cloud-python@main`.

So basically, just updating the librarian commit to reference one on the
`googleapis@preview` branch.