RFR: 8319725: G1: Subtracting virtual time from wall time after JDK-8319204

[Aleksey Shipilev]

Noticed this when looking at the [JDK-8319204](https://bugs.openjdk.org/browse/JDK-8319204) changeset: we end subtracting `elapsedVTime()` vs `elapsedTime()`, which looks incorrect, as it does math on two unrelated quantities.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16564/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16564&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319725
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16564.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16564/head:pull/16564

PR: https://git.openjdk.org/jdk/pull/16564