Remove StringCharIntrinsics flag after JDK-8138651 is fixed

We need to remove the workaround fix for JDK-8138651:

725 case vmIntrinsics::_putCharStringU:
726 // Until JDK-8138651 is fixed, we have to rely on a special flag to
727 // disable these intrinsics for experiments.
728 if (!StringCharIntrinsics) return true;

http://hg.openjdk.java.net/jdk9/sandbox/hotspot/rev/f192cdd66c6d