RFR: Remove redundant initialization code

[Kelvin Nilsen]

This code and the assertions that acompany it are subject to races with certain GC worker threads which are also capturing top_at_mark_start() and may capture it at a different position than this thread does.

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Commit messages:
 - Remove redundant initialization code

Changes: https://git.openjdk.org/shenandoah/pull/267/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=267&range=00
  Stats: 12 lines in 1 file changed: 0 ins; 11 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/267.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/267/head:pull/267

PR: https://git.openjdk.org/shenandoah/pull/267