RFR: Remove log message that could access an out of bounds region

[William Kemper]

The message attempts to reference the region containing the top of the plab. When the plab is full and is at the end of the last region in the heap, there will be no containing region. Doesn't seem worth the trouble to special case for a debug message.

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Commit messages:
 - Remove log message that could access an out of bounds region

Changes: https://git.openjdk.java.net/shenandoah/pull/66/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=66&range=00
  Stats: 6 lines in 1 file changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/66.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/66/head:pull/66

PR: https://git.openjdk.java.net/shenandoah/pull/66