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

[Amit Kumar]

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 :)

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Commit messages:
 - FOUND_CORES variable could be removed

Changes: https://git.openjdk.org/jdk/pull/12285/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12285&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298424
  Stats: 13 lines in 1 file changed: 2 ins; 8 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12285.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12285/head:pull/12285

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