[libSyntax] Delete discardRecordedNode

It doesn't do anything anymore and we shouldn't have it, so let's just
remove it.

tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp

@@ -197,10 +197,6 @@ private:
     return None;
   }
 
-  void discardRecordedNode(OpaqueSyntaxNode node) override {
-    // FIXME: This method should not be called at all.
-  }
-
   std::pair<size_t, OpaqueSyntaxNode>
   lookupNode(size_t lexerOffset, SyntaxKind kind) override {
     auto NodeLookup = getNodeLookup();