Code Quality: Remove showSemanticHeadings feature flag code

[timarney]

Context

This feature has been enabled by default in v1.23.0.

Feature PR #11888

Summary

Removes showSemanticHeadings feature flag

Relevant Technical Choices

To-do

User-facing changes

Testing Instructions

• This is a non-user-facing change and requires no QA

This PR can be tested by following these steps:

1. Navigate to a Story within the Editor.
2. Add multiple text elements with variying font sizes.
3. Note that the feature is on by default ... You should be able to see in the Style section of the sidebar, underneath a new Accessibility tab, the type of semantic heading.

Reviews

Does this PR have a security-related impact?

No

Does this PR change what data or activity we track or use?

No

Does this PR have a legal-related impact?

No

Checklist

• This PR addresses an existing issue and I have linked this PR to it in ZenHub
• I have tested this code to the best of my abilities
• I have verified accessibility to the best of my abilities (docs)
• I have verified i18n and l10n (translation, right-to-left layout) to the best of my abilities
• This code is covered by automated tests (unit, integration, and/or e2e) to verify it works as intended (docs)
• I have added documentation where necessary
• I have added a matching Type: XYZ label to the PR

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Fixes #11996

[googleforcreators-bot]

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