Remove support for non-async immediate tasks

[pedro-psb]

I forget to remove this for 3.85, as announced in #6155
Because we just released 3.85 and are working on related issues, I'm doing it a little late and backporting it.

It's not clear if plugins have to take any action about that. I don't think so...

[pedro-psb]

I'm out tomorrow (14 aug). Anyone feel free to take this over.

[patchback]

Backport to 3.85: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 4a26f97 on top of patchback/backports/3.85/4a26f973f3f50e5acfaf9667d502cf5b61ebb6aa/pr-6857

Backporting merged PR #6857 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pulp/pulpcore.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.85/4a26f973f3f50e5acfaf9667d502cf5b61ebb6aa/pr-6857 upstream/3.85

4. Now, cherry-pick PR Remove support for non-async immediate tasks #6857 contents into that branch:

   $ git cherry-pick -x 4a26f973f3f50e5acfaf9667d502cf5b61ebb6aa

   If it'll yell at you with something like fatal: Commit 4a26f973f3f50e5acfaf9667d502cf5b61ebb6aa is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 4a26f973f3f50e5acfaf9667d502cf5b61ebb6aa

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Remove support for non-async immediate tasks #6857 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.85/4a26f973f3f50e5acfaf9667d502cf5b61ebb6aa/pr-6857

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[mdellweg]

Manual backport: #6860