Remove testing/android_background_image which does not run/exist on CI. (#51815)

I am guessing this just either served it's purpose (flutter/flutter#26654, flutter/flutter#26728) or didn't and was never followed up. I can't find any examples of it running on CI - I suspect if we want a test like this, we're better off just adding it to `scenario_app` versus creating more 1-off Android integration test environments.

Author: matanlurey

BUILD.gn

@@ -155,11 +155,6 @@ group("flutter") {
       (is_ios || is_android)) {
     public_deps += [ "//flutter/testing/scenario_app" ]
   }
-
-  if (is_android && flutter_runtime_mode == "profile" &&
-      current_cpu == "arm64") {
-    public_deps += [ "//flutter/testing/android_background_image" ]
-  }
 }
 
 group("unittests") {