Remove unnecessary 'visitDecl' default cases.

The default cases are unnecessary and provide no benefits so they should
just be removed.

tools/swift-ide-test/ModuleAPIDiff.cpp

@@ -781,10 +781,6 @@ public:
     return Result;
   }
 
-  void visitDecl(Decl *D) {
-    // FIXME: maybe don't have a default case
-  }
-
   void visitStructDecl(StructDecl *SD) {
     auto ResultSD = std::make_shared<sma::StructDecl>();
     ResultSD->Name = convertToIdentifier(SD->getName());