Enable the serialization of sil_vtables by default and completely remove the -sil-serialize-vtables option

Only sil_vtables of public classes with fixed layout are serialized.

lib/Frontend/CompilerInvocation.cpp

@@ -1208,9 +1208,6 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_sil_merge_partial_modules))
     Opts.MergePartialModules = true;
 
-  Opts.SILSerializeVTables |=
-    Args.hasArg(OPT_sil_serialize_vtables);
-
   // Parse the optimization level.
   // Default to Onone settings if no option is passed.
   IRGenOpts.Optimize = false;