RFR (XXS) 8213767: Remove the -Xconcurrentio flag and associated code

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8213767
CSR: https://bugs.openjdk.java.net/browse/JDK-8213768

webrev: http://cr.openjdk.java.net/~dholmes/8213767/webrev/


This flag dates back to the JDK 1.2/1.3 days and was useful primarily on 
Solaris where it enabled use of LWP synchronization. But that became the 
default in JDK 1.4. The flag serves no useful purpose and is unlikely to 
have been used for many years.

Thanks,
David