Remove (almost all of) //sky/engine/web

This layer is no longer needed now that the new world is gone.

TBR=eseidel@google.com

Review URL: https://codereview.chromium.org/1210153009.

engine/src/flutter/BUILD.gn

@@ -8,7 +8,6 @@ group("sky") {
   deps = [
     "//services/debugger",
     "//sky/engine/platform:platform_unittests",
-    "//sky/engine/web:sky_unittests",
     "//sky/engine/wtf:unittests",
     "//sky/sdk/example",
     "//sky/tools/imagediff",