RFR: 8277990: NMT: Remove NMT shutdown capability

[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

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Commit messages:
 - Fix comment
 - Fix windows
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/6640/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6640&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277990
  Stats: 446 lines in 18 files changed: 31 ins; 349 del; 66 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