RFR: 8303054: Remove unused variables in GCTraceTimeLoggerImpl::log_end

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/12711/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12711&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303054
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12711.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12711/head:pull/12711

PR: https://git.openjdk.org/jdk/pull/12711