RFR: 8273386: Remove duplicated code in G1DCQS::abandon_completed_buffers

[Kim Barrett]

Please review this small change to remove some redundant code in
G1DirtyCardQueueSet::abandon_completed_buffers.  After the two removed lines
it calls take_all_completed_buffers, which begins with those same two lines.

Testing:
Local (linux-x64) hotspot:tier1 with -XX:+UseG1GC.

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Commit messages:
 - remove redundant code

Changes: https://git.openjdk.java.net/jdk/pull/5382/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5382&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273386
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5382.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5382/head:pull/5382

PR: https://git.openjdk.java.net/jdk/pull/5382