Revert "Reinstate "Compile StdlibUnittest with -sil-serialize-all."""
Revert "Reinstate "Compile StdlibUnittest with -sil-serialize-all."""

This reverts commit 29214253e58dc59d4ab5444ffb1a928f783e3f15.

This change massively regresses test execution times for DebugAssert builds, where we won't currently benefit from this added validation.
Moving forward, we'll scope this exclusively to optimized builds.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift