RFR (S) 8223657: Remove unused THREAD argument from SymbolTable functions

[coleen.phillimore at oracle.com]

Summary: also made lookup and lookup_only functions private to 
SymbolTable.  External callers use new_symbol or probe.

Ran hs-tier1-3.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8223657.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8223657

Thanks,
Coleen