RFR (M) 8213753: SymbolTable is double walked during class unloading and clean up table timing in do_unloading

[coleen.phillimore at oracle.com]

Summary: remove gc timing for short runtime cleanup triggering; make 
symbol table cleaning triggered automatically on unloading

Ran runThese with all Oracle GCs and got similar numbers of symbols 
unloaded.  Also ran tier1-5.

See bug for more information.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8213753.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8213753

Thanks,
Coleen