RFR: JDK-8260222: remove unused _thread member SymbolTableLookup

[Matthias Baesken]

The class SymbolTableLookup has an unused member _thread. This could be removed.
See also the related sonar finding (claiming an uninitialized field) :

https://sonarcloud.io/project/issues?id=jdk&open=AXaE0c2j8L9hkQskF8y_&resolved=false&types=BUG

Thanks, Matthias

-------------

Commit messages:
 - JDK-8260222

Changes: https://git.openjdk.java.net/jdk/pull/2180/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2180&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260222
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2180.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2180/head:pull/2180

PR: https://git.openjdk.java.net/jdk/pull/2180