Remove all SSE references [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do and why?

Removes code for the removed Static Site Editor feature.

NOTEs:

• The metrics and usage_data config and files will be deleted in a follow-up MR. Here's the issue for that: Remove metrics and usage_data config for Static... (#362977 - closed)
• Final feature flag cleanup and any other remaining references (e.g. rubocop, CODEOWNERS) will be cleaned up in Delete all references to static site editor fea... (#362152 - closed)

Related: Deprecate Static Site Editor in 14.7 (#347137 - closed) (deprecation issue) Related: Remove metrics and usage_data config for Static... (#362977 - closed) (metrics and usage_data removal issue) Related: Remove Static Site Editor code (#352505 - closed) (removal issue)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.