Remove sun.misc.Unsafe.{shouldBeInitialized,ensureClassInitialized}

Unsafe.{shouldBeInitialized,ensureClassInitialized} are deprecated for removal since JDK 15. It's time to remove them. Lookup.ensureInitialized(Class) was added in Java 15 as a standard API so no need to use these Unsafe.ensureClassInitialized anymore. Unsafe.shouldBeInitialized isn't reliable when multiple threads may initialise the class, users of that method should look at Lookup.ensureInitialized too.