Remove mojo_bindings.js from WPT

Bug: 1122581
Change-Id: I80e42994300d4785a8b79ad6d6f3bcda61441dcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386391Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803505}

third_party/blink/web_tests/external/wpt/resources/chromium/README.md

-This directory contains Chromium-specific test resources.
+This directory contains Chromium-specific test resources, including mocks for
+test-only APIs implemented with
+[MojoJS](https://chromium.googlesource.com/chromium/src/+/master/mojo/public/js/README.md).
 
-The files `mojo_bindings.js` and `*.mojom.js` are manually copied from the
-Chromium build process's generated files and should not be edited manually.
+Please do **not** copy `*.mojom.js` into this directory. Follow this doc if you
+want to add new MojoJS-backed mocks:
+https://chromium.googlesource.com/chromium/src/+/master/docs/testing/web_platform_tests.md#mojojs

third_party/blink/web_tests/external/wpt/resources/chromium/mojo_bindings.js.headers

-Content-Type: text/javascript; charset=utf-8