Doc: remove `role="group"` from some split drop* buttons

Description

• Remove role="group" from the split dropstart button in the documentation
• Remove role="group" from visual JS tests

Motivation & Context

Since v4.6.1 (at least), the split dropstart button example in the documentation has a role="group". But this is the only example having it.

We don't see this role for split dropend and split dropup, and "normal" split buttons.

This PR brings more consistency in the examples by removing this extra-role.

⚠ However, I don't have the expertise to say if it could be useful in terms of a11y. In this case, this PR should rather be updated to add this role wherever it is useful.

Note: Since it is observed in v4.x as well, whatever the final modification will be, I suppose it could be backported in v4 as well.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• All new and existing tests passed

Related issues

N/A

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