RFR: 8262259: Remove unused variable in MethodLiveness::BasicBlock::compute_gen_kill_single

[Zhengyu Gu]

Please review this trivial patch that removes unused/dead local variable.

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Commit messages:
 - Merge branch 'master' into JDK-8262259-rm-unused-localNum
 - JDK-8262259

Changes: https://git.openjdk.java.net/jdk/pull/2698/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2698&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262259
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2698.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2698/head:pull/2698

PR: https://git.openjdk.java.net/jdk/pull/2698