[build] Don't reset 'swiftlib_link_flags_all' unnecessarily

Author: finagolfin

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -1437,7 +1437,6 @@ function(add_swift_target_library name)
 
       # Add PrivateFrameworks, rdar://28466433
       set(swiftlib_c_compile_flags_all ${SWIFTLIB_C_COMPILE_FLAGS})
-      set(swiftlib_link_flags_all ${SWIFTLIB_LINK_FLAGS})
 
       # Add flags to prepend framework search paths for the parallel framework
       # hierarchy rooted at /System/iOSSupport/...