Remove `_alwaysRunsAtForegroundPriority` for wkwebview because it's a private API

Apps that use a private API in the app store might not always get rejected, but the latest version of Xcode does prompt with a warning during the uploading process that we're using a private API.

Use of private APIs is against the Apple developer agreement and should be removed or compiled out with a macro/flag at the very least (by default).