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

This reverts commit adc92ba.

It caused a fail of stdlib/AtomicInt.swift on linux.

Author: Erik Eckstein

stdlib/private/StdlibUnittest/CMakeLists.txt

@@ -48,7 +48,6 @@ add_swift_library(swiftStdlibUnittest SHARED IS_STDLIB
 
   PRIVATE_LINK_LIBRARIES ${swift_stdlib_unittest_private_link_libraries}
   SWIFT_MODULE_DEPENDS ${swift_stdlib_unittest_module_depends}
-  SWIFT_COMPILE_FLAGS -Xfrontend -sil-serialize-all
   FRAMEWORK_DEPENDS ${swift_stdlib_unittest_framework_depends}
   INSTALL_IN_COMPONENT stdlib-experimental)