RFR: 8289520: G1: Remove duplicate checks in G1BarrierSetC1::post_barrier

[Albert Mingkun Yang]

Simple change of removing effectively dead code.

Test: tier1-3

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Commit messages:
 - g1-post-barrier

Changes: https://git.openjdk.org/jdk/pull/9333/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9333&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289520
  Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9333.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9333/head:pull/9333

PR: https://git.openjdk.org/jdk/pull/9333