RFR: JDK-8296443 NMT: Remove cmdline_tracking_level handling code

[Thomas Stuefe]

Trivial cleanup.

NMT tracking level had been shadowed by `_cmdline_tracking_level` to notice tracking level degradation due to resource exhaustion. All that tracking level handling has simplified with [8256844](https://bugs.openjdk.org/browse/JDK-8256844). Coding can be removed.

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Commit messages:
 - JDK-8296443-NMT-Remove-cmdline_tracking_level

Changes: https://git.openjdk.org/jdk/pull/11006/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11006&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296443
  Stats: 15 lines in 3 files changed: 1 ins; 12 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11006.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11006/head:pull/11006

PR: https://git.openjdk.org/jdk/pull/11006