[dsymutil] Remove stale comment (NFC)

The comment was no longer relevant after r362621.

llvm-svn: 363008

tools/dsymutil/MachOUtils.h

@@ -25,7 +25,6 @@ namespace MachOUtils {
 
 struct ArchAndFile {
   std::string Arch;
-  // Optional because TempFile has no default constructor.
   std::unique_ptr<llvm::sys::fs::TempFile> File;
 
   llvm::Error createTempFile();