RFR: Remove unnecessary checks identified by reviewer

[Kelvin Nilsen]

A previous pull request was integrated before all of the reviewer
comments had been addressed.  This commit addresses the lingering
request.

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Commit messages:
 - Remove unnecessary checks identified by reviewer

Changes: https://git.openjdk.org/shenandoah/pull/146/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=146&range=00
  Stats: 37 lines in 1 file changed: 0 ins; 8 del; 29 mod
  Patch: https://git.openjdk.org/shenandoah/pull/146.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/146/head:pull/146

PR: https://git.openjdk.org/shenandoah/pull/146