docs: Delete instructions to run non-existent soundness.sh #763
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Motivation
scripts/soundness.sh was deleted in #604 which moved CI to GitHub Actions.
Credit to @heckj, who found the same problem in apple/swift-container-plugin#96.
Modifications
Remove the instruction to run
scripts/soundness.shfromCONTRIBUTING.md.Result
Documentation no longer refers to nonexistent scripts.
Test Plan
No functional change.