fix(accordion): simplify header sizing, remove unmanaged custom property

[Westbrook]

Description

fixes #1033

• simplify calculation of the Accordion heading to rely on %s rather than --spectrum-accordion-item-height-actual
• remove use of --spectrum-accordion-item-height-actual custom property (which was unmanaged across various scales)

How and where has this been tested?

• How this was tested: Steps in [Accordion] Item hight not using a controlled CSS Custom Property #1033 and the locally built demo site.
• Browser(s) and OS(s) this was tested with: Chrome on macOS

Screenshots

To-do list

• If my change impacts other components, I have tested to make sure they don't break.
• If my change impacts documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.

• This pull request is ready to merge.

[Westbrook]

This looks like it's failing the visual regression internal to the CI. It's not clear in the output there if it's just my one pattern, as expected, or not. Is there anything I can/need to do about that being I expect there to be one pattern with failure(s) here?

[Westbrook]

closing in preference of #1048