RFR: 8235654: JFR leak profiler should not trace through the StringTable

[erik.osterlund at oracle.com]

Hi,

The JFR leak profiler finds strong reference chains to object samples. 
Weirdly enough, it walks the StringTable roots. But these oops are 
phantomly reachable. So they should arguably be excluded from the root 
snapshot.This incorrect use of StringTable::oops_do is the only use of 
StringTable::oops_do, so I'm removing that function.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8235654

Webrev:
http://cr.openjdk.java.net/~eosterlund/8235654/webrev.00/

Thanks,
/Erik