RFR: 8318474: Fix memory reporter for thread_count

[Johan Sjölen]

Hi,

[JDK-8315362](https://bugs.openjdk.org/browse/JDK-8315362) did the right thing and always updated the `_thread_count` regardless of the 'mode' that we are in, but we forgot about this assert. So, let's get rid of the assert.

@tstuefe, @gerard-ziemski, I was wondering if we can't actually just use the `_thread_count` on AIX now when printing the thread count, shouldn't that be correct now? Regardless, let's not have this assert, as it breaks NMT tests on AIX.

@MBaesken, would you mind trying this patch out? It's on top of current master, so you can check it out directly.

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Commit messages:
 - Remove unnecessary assert

Changes: https://git.openjdk.org/jdk/pull/16255/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16255&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318474
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16255.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16255/head:pull/16255

PR: https://git.openjdk.org/jdk/pull/16255