JDK 10 RFR of 8189963: Remove version of FileChannelImpl::open without the 'direct' parameter

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8189963
http://cr.openjdk.java.net/~bpb/8189963/webrev.00/

This minor change removes a small amount of code by eliminating one method and changing the call sites of the method to invoke instead a more general, equivalent method.

Thanks,

Brian