chore: Remove Marketing issue templates migrated to ai_initiative_marketing

Summary

Removes the 5 Marketing issue templates from project/ai_initiative now that the AI Marketing Initiative has its own repo. These templates were duplicated into project/ai_initiative_marketing and should live there, not here.

Removed:

• Marketing - Blog Entry.md
• Marketing - Case Study.md
• Marketing - Generic.md
• Marketing - Video.md
• Marketing - Webinar.md

The Initiative - * and Funding - Monthly templates are untouched.

Related

• Migration tracking: ai_initiative_marketing#122 (closed)
• Supersedes the closed sprint-stats MR !12 (closed) (the marketing tooling moved to ai_initiative_marketing).

Drafted with the assistance of AI (Claude Code / Opus 4.8).