Remove support for pre-Java 6 non-JVM-wide file locking

In Java 6 a system property sun.nio.ch.disableSystemWideOverlappingFileLockCheck was added to control file locking behavior, specifically to enable suppression of JVM-wide file locking so as to provide compatibility with prior Java versions. The old behavior was constrained to check for locks obtained on the channel instance only, not JVM-wide which is what was actually specified. At this point the old behavior should no longer be needed and support for the property should be removed.