Add note to remove TxResult proto

As Sergio pointed out in 3e31aa6f583cdc71e208ed03a82f1d804ec0de49, this
proto message can probably be removed. We should do this in a separate
PR.

Signed-off-by: Thane Thomson <connect@thanethomson.com>

proto/tendermint/abci/types.proto

@@ -424,6 +424,10 @@ message ExecTxResult {
 // TxResult contains results of executing the transaction.
 //
 // One usage is indexing transaction results.
+// TODO(thane): Remove this struct. It's effectively only used in
+//              types/events.go#EventDataTx and these fields could easily be
+//              introduced directly into that struct. There doesn't seem to be
+//              any other use for this struct via any other interface.
 message TxResult {
   int64        height = 1;
   uint32       index  = 2;