RFR: JDK-8289182: NMT: MemTracker::baseline should return void

[Thomas Stuefe]

Small cleanup.

Since MemTracker::baseline() only returns true, it can revert to void, and error handling can be removed from all callers.

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Commit messages:
 - let MemTracker::baseline return void

Changes: https://git.openjdk.org/jdk/pull/9288/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9288&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289182
  Stats: 47 lines in 8 files changed: 0 ins; 11 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/9288.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9288/head:pull/9288

PR: https://git.openjdk.org/jdk/pull/9288