Revert "[deserialization] Deserialize WitnessTables/VTables in SILModule, not in SILLinking."

This reverts commit r14623. I will recommit this when I reenable deserialization
of witness tables/vtables. This will enable the deserialization of SILFunctions
by default from the stdlib by avoiding the lack of external witness
tables/vtables.

Swift SVN r14786

lib/SIL/SILModule.cpp

@@ -84,8 +84,7 @@ SILModule::SILModule(Module *SwiftModule)
   TypeListUniquing = new SILTypeListUniquingType();
   SILLoader = SerializedSILLoader::create(getASTContext(), this,
                                           Callback.get());
-  SILLoader->getAllVTables();
-  SILLoader->getAllWitnessTables();
+
 }
 
 SILModule::~SILModule() {