RFR: 8277990: NMT: Remove NMT shutdown capability [v3]

[Zhengyu Gu]

> 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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Aleksey's comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6640/files
  - new: https://git.openjdk.java.net/jdk/pull/6640/files/58d22421..30aa1a52

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6640&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6640&range=01-02

  Stats: 6 lines in 3 files changed: 0 ins; 2 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6640.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6640/head:pull/6640

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