fix: assign correct role attribute in popup menu item

[barmac]

Related camunda/camunda-modeler#4383

Proposed Changes

This removes incorrect aria-role attribute from the popup menu item. Note that the <li> element has an implicit role of listitem and a role=button element as a direct child of a list is not allowed.

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[barmac]

I removed the role attribute to keep the changes as minimal as possible. aria-role was already incorrect, so there are no changes in the a11y tree with this PR.