RFR(XS): 8213438: Remove ClearResponsibleAtSTW

[Mikael Vidstedt]

Please review this small change which removes an “unused” enum/value and some effectively dead code.

bug: https://bugs.openjdk.java.net/browse/JDK-8213438 <https://bugs.openjdk.java.net/browse/JDK-8213438>
webrev: http://cr.openjdk.java.net/~mikael/webrevs/8213438/webrev.00/open/webrev/ <http://cr.openjdk.java.net/~mikael/webrevs/8213438/webrev.00/open/webrev/>

* Background (from jbs)

The ClearResponsibleAtSTW enum value in src/hotspot/share/runtime/synchronizer.cpp is set to 0 and only used in a single place: 

if (ClearResponsibleAtSTW) mid->_Responsible = NULL; 

Since the value is 0/false that code is effectively dead. The enum and the dead code should be removed.

* Testing

Built linux-x64-debug locally. Will run tier1 just to be extra sure.

Cheers,
Mikael