GH-7103: API for removing toolbar item.

[kittaakos]

What it does

This PR adds API for removing toolbar contributions. Closes #7103.

How to test

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

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[westbury]

This looks good to me. In general, for removals, we need to ensure that contributions made by plugins can also be removed, which means keeping a list of removed items that is checked when plugins are installed. However I did not see anything in the plugin API that can add toolbar items so I think we are fine there.

I tested this by removing the ''scm.viewmode.list' item (contributed by the scm package) by calling unregisterItem from the git package.

The second commit to remove the unnecessary async looks good to me too.

[paul-marechal]

which means keeping a list of removed items that is checked when plugins are installed. However I did not see anything in the plugin API that can add toolbar items so I think we are fine there.

This can be added later if the need comes up.