Should we remove the implicit role from `summary`?

The [summary](https://www.w3.org/TR/html-aria/#el-summary) element in ARIA 1.2 no longer has an implicit role. However it does note that browsers may still give it an implicit role. We should investigate if we should keep the `button` role as our implicit role for summary or remove it and leave it as having no implicit role.

Start by comparing axe-core's current implicit-role handling for the summary element with the ARIA 1.2 summary mapping linked in the issue. Determine whether the button role should remain or be removed, and document the decision and resulting behavior; no source files or tests are identified in the issue.

Written by the indexing model from the issue text.