Remove CanvasProxy along with related concepts and complexities

The CanvasProxy feature is being removed to make room for a newer
incarnation known as OffscreenCanvas, which will be added soon.
Draft Proposal: https://wiki.whatwg.org/wiki/OffscreenCanvas.
There are currently no known implementations of CanvasProxy.

Summary of changes:

* Remove CanvasProxy and all canvas methods related to CanvasProxy
* Remove the notion of a CanvasRenderingContext2D being in a proxied
  state.
* Remove the notion of a CanvasRenderingContext2D being in an indirect
  state.
* Remove the notion of a "scratch bitmap" from CanvasRenderingContext2D
  and use a single concept of an "output bitmap" everywhere.
* Remove the notion of a "list of pending user interface actions"
* Remove the notion canvas context binding: no more standalone
  contexts, no more setContext method on canvas.
* Remove the commit() method of CanvasRenderingContext2D.
* Remove the CanvasRenderingContext2D constructor.
* Remove the notion of concept-canvas-context-bitmap-mode, canvii
  now always behave as what was previous described as "fixed" mode.
* Rename bitmap modes "direct-2d" and "direct-webgl" to just "2d" and
  "webgl" respectively.
* The hit region list, alpha flag, and origin-clean flag are now
  internal attributes of CanvasRenderingContext2D, rather than
  attributes of the scratch/output bitmaps.
* CanvasRenderingContext2D and its sub-interfaces are no longer
  exposed in Workers.

PR: #790