RFR: 8277990: NMT: Remove NMT shutdown capability

[Daniel D.Daugherty]

On Wed, 1 Dec 2021 16:00:09 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> NMT shutdown functionality is a remnant of its first implementation, which could consume excessive amount of memory, therefore, it needed capability to shut it self down to ensure health of JVM. This is no longer a case for current implementation.
> 
> After JDK-8277946, there is no longer a use, so it can be removed.
> 
> Test:
> - [x] hotspot_nmt
> - [x] tier1 with NMT on

Hmmm... sounds like a ReadersWriter lock to me... Any number of Readers and a single
Writer after all the Readers have released... It has surprised me before that HotSpot does
not have such a useful mechanism...

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PR: https://git.openjdk.java.net/jdk/pull/6640