RFR: 8282878: Removed _JavaThread from PhaseTraceTime

[Aleksey Shipilev]

On Wed, 9 Mar 2022 11:49:42 GMT, Alex Blewitt <duke at openjdk.java.net> wrote:

> The `PhaseTraceTime` class has a private `JavaThread _thread` field which doesn't appear to be referenced anywhere, and builds fine without it. It's not clear if this can be removed but it builds cleanly when not present.

I believe this is trivial too.

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PR: https://git.openjdk.java.net/jdk/pull/7756