RFR: 8275714: G1: remove unused variable in G1Policy::transfer_survivors_to_cset

[Albert Mingkun Yang]

Trivial change of removing unused variables.

Test: build

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Commit messages:
 - g1policy

Changes: https://git.openjdk.java.net/jdk/pull/6062/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6062&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275714
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6062.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6062/head:pull/6062

PR: https://git.openjdk.java.net/jdk/pull/6062