RFR: Remove assert that plab allocation must succeed

[William Kemper]

In some cases, plab alignment will eat into the usable free memory for the region so we cannot assert that the allocation must succeed.

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Commit messages:
 - Remove assert that plab allocation must succeed

Changes: https://git.openjdk.java.net/shenandoah/pull/98/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=98&range=00
  Stats: 11 lines in 1 file changed: 0 ins; 1 del; 10 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/98.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/98/head:pull/98

PR: https://git.openjdk.java.net/shenandoah/pull/98