[api-minor] Remove the WebGL implementation

Reasons for the removal include:
 - This functionality was always somewhat experimental and has never been enabled by default, partly because of worries about rendering bugs caused by e.g. bad/outdated graphics drivers.

 - After the initial implementation, in PR 4286 (back in 2014), no additional functionality has been added to the WebGL implementation.

 - The vast majority of all documents do not benefit from WebGL rendering, since only a couple of *specific* features are supported (e.g. some Soft Masks and Patterns).

 - There is, and has always been, *zero* test-coverage for the WebGL implementation.

 - Overall performance, in the PDF.js library, has improved since the experimental WebGL implementation was added.

Rather than shipping unused *and* untested code, it seems reasonable to simply remove the WebGL implementation for now; thanks to version control it's always possible to bring back the code should the need ever arise.

web/app_options.js

@@ -70,11 +70,6 @@ const defaultOptions = {
     value: true,
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
-  enableWebGL: {
-    /** @type {boolean} */
-    value: false,
-    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
-  },
   externalLinkRel: {
     /** @type {string} */
     value: "noopener noreferrer nofollow",