Remove unused CONTRIBUTORS file as well as references to it

[jsheunis]

Updates:

• CONTRIBUTORS
• MANIFEST.in

Closes https://github.com/datalad/datalad-extension-template/issues/84

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (81dcfe5) 92.68% compared to head (3dfc2ca) 92.68%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #87 +/- ## ======================================= Coverage 92.68% 92.68% ======================================= Files 5 5 Lines 41 41 ======================================= Hits 38 38 Misses 3 3 ```

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[adswa]

I have voiced a few reasons against this PR in the original issue. In summary, these were:

• the fact that it is unused makes sense insofar that this is a template for others to use
• it is standard practice to include it in python packages according to the popular cookiecutter template, and GitHub includes this file in its list of community guidelines (a checklist projects can fulfill in their settings that aims to promote community-orientation in projects)

[mslw]

I understand where the PR is coming from in that the contributors file is unused (or rather: not used actively, i.e. not updated) across several DataLad projects, including core (3 yrs ago) and next, where zenodo.json became the de facto contributor list.

But the arguments presented by @adswa are compelling, and I would vote to keep the file in the template.

[adswa]

In the spirit of not leaving forgotten clutter around, I'll close this PR - but if there are new viewpoints that speak for this PR, let's reopen this and rekindle the discussion.