Remove the disableCanvasToImageConversion functionality from PDFThumbnailView (PR 7029 follow-up)

This functionality was originally implemented in PR 7029; however it's not, nor has it ever been, used as far as I can tell.[1]
Note in particular that the default viewer does not expose either a preference or even an option with which `disableCanvasToImageConversion` can be toggled, and source-code modification is thus required.

Furthermore, note also that we have multiple other instances of `canvas`-data accesses in both the `src/display/canvas.js` and `src/display/text_layer.js` files. If any of those are blocked, by e.g. browser settings, there will be outright rendering bugs and non-working thumbnails thus seem like a very small issue in the grand scheme of things; hence why I'm suggesting that we remove the unused `disableCanvasToImageConversion` functionality.

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[1] For the Tor use-case mentioned in issue 7026, I *believe* that the solution was to white-list `canvas`-data accesses for its built-in PDF Viewer.

web/pdf_thumbnail_viewer.js

@@ -209,7 +209,6 @@ class PDFThumbnailViewer {
             linkService: this.linkService,
             renderingQueue: this.renderingQueue,
             checkSetImageDisabled,
-            disableCanvasToImageConversion: false,
             l10n: this.l10n,
           });
           this._thumbnails.push(thumbnail);