refactor(test): remove what-comment from makeFakePdfjsLib

The name already implies it's a fake; the comment described what, not why.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts

@@ -24,7 +24,6 @@ function makeFakePdfjsLib() {
 			})
 		}),
 		TextLayer: TextLayerMock
-		// Partial fake: only the methods used by usePdfRenderer are implemented
 	} as unknown as typeof import('pdfjs-dist');
 }