Remove gradle_non_android_plugin_test test and builder when no longer needed in stable

Delete [gradle_non_android_plugin_test](https://github.com/flutter/flutter/blob/master/dev/devicelab/bin/tasks/gradle_non_android_plugin_test.dart) and builder when https://github.com/flutter/flutter/pull/80161 is in stable. See also todos in https://github.com/flutter/infra/pull/411.