docs: remove backward compatibility comments from ebook handler

- Removed comment about 'Ebook notes handlers (backward compatibility using views)'
- Removed comment references to non-existent GetEbookNotes and GetEbookHighlights
- Cleaned up misleading legacy documentation

This is part of legacy code cleanup Phase 1.
Phase 1: Documentation Cleanup

internal/handlers/ebook.go

@@ -1291,10 +1291,6 @@ func (h *Handler) DeleteMediaHighlight(c echo.Context) error {
 	return c.NoContent(http.StatusNoContent)
 }
 
-// Ebook notes handlers (backward compatibility using views)
-
-// GetEbookNotes handles GET /api/ebooks/:id/notes
-// GetEbookHighlights handles GET /api/ebooks/:id/highlights
 // SearchMediaItems handles GET /api/media-items/search
 // Performs partial matching search with fuzzy fallback if no results found
 func (h *Handler) SearchMediaItems(c echo.Context) error {