RFR (T) 8213399: DecoderLocker is unused

[coleen.phillimore at oracle.com]

Summary: remove DecoderLocker

See bug for history and more info.   Ran mach5 tier1 and 
runtime/ErrorHandler tests on linux and windows.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8213399.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8213399

Thanks,
Coleen