RFR: 8298424: Remove redundant FOUND_CORES variable in build-performance.m4

[Tyler Steele]

On Mon, 30 Jan 2023 08:42:14 GMT, Amit Kumar <duke at openjdk.org> wrote:

> Beside any specific reason which I'm unaware of, IMO FOUND_CORES variable serves no purpose. So probably we could remove it, without any issue. 
> 
> Any suggestion/thoughts are appreciated :)

Marked as reviewed by tsteele (Committer).

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PR: https://git.openjdk.org/jdk/pull/12285