Remove IfConfigDecl from the AST

The swift-syntax tree retains information about the parsed #if
regions. Drop it from the semantic AST.

lib/IDE/SyntaxModel.cpp

@@ -987,8 +987,6 @@ ASTWalker::PreWalkAction ModelASTWalker::walkToDeclPre(Decl *D) {
     }
     pushStructureNode(SN, VD);
 
-  } else if (isa<IfConfigDecl>(D)) {
-    // Note: nothing to do.
   } else if (auto *EnumCaseD = dyn_cast<EnumCaseDecl>(D)) {
     SyntaxStructureNode SN;
     setDecl(SN, D);