RFR: 8260256: Leftover Thread uses after JDK-8223657

[Aleksey Shipilev]

JDK-8223657 cleaned up Thread arguments in SymbolTable, but there are some leftovers. Notably, SonarCloud reports that SymbolTableLookup::_thread is not initialized. I looked for `Thread` usages in `symbolTable.cpp`, and it seems only these two are missing. Maybe @coleenp can spot even more omissions somewhere.

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Commit messages:
 - 8260256: Leftover Thread uses after JDK-8223657

Changes: https://git.openjdk.java.net/jdk/pull/2188/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2188&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260256
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2188.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2188/head:pull/2188

PR: https://git.openjdk.java.net/jdk/pull/2188