Stop including isOffscreenCanvasSupported in the "StartRenderPage" message

With the previous commit this is now completely unused in API, hence it can be removed. This is done in a separate commit to make it easier to re-instate it, would the need ever arise.

src/core/document.js

@@ -417,8 +417,6 @@ class Page {
           this.resources,
           this.nonBlendModesSet
         ),
-        isOffscreenCanvasSupported:
-          this.evaluatorOptions.isOffscreenCanvasSupported,
         pageIndex: this.pageIndex,
         cacheKey,
       });