RFR: Remove assert that plab allocation must succeed

[Kelvin Nilsen]

On Thu, 11 Nov 2021 18:44:20 GMT, William Kemper <wkemper at openjdk.org> wrote:

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

Marked as reviewed by kdnilsen (Committer).

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