CMake: do not link StringProcessing into StdlibUnittest

Since #64187 we are not importing automatically StringProcessing when
building the stdlib -- this is especially important for StdlibUnittest,
since it needs to run on older OSes.

Addresses rdar://157622116

Author: edymtt

stdlib/private/StdlibUnittest/CMakeLists.txt

@@ -37,9 +37,6 @@ if (SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY)
   list(APPEND swift_stdlib_unittest_link_libraries "swift_Concurrency")
   list(APPEND swift_stdlib_unittest_modules "_Concurrency")
 endif()
-if (SWIFT_ENABLE_EXPERIMENTAL_STRING_PROCESSING)
-  list(APPEND swift_stdlib_unittest_modules "_StringProcessing")
-endif()
 
 add_swift_target_library(swiftStdlibUnittest ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
   # This file should be listed the first.  Module name is inferred from the