RFR: 8285934: Remove unimplemented MemTracker::init_tracking_level

[Albert Mingkun Yang]

Trivial change of removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/8469/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8469&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285934
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8469.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8469/head:pull/8469

PR: https://git.openjdk.java.net/jdk/pull/8469