walletrpc: add new RemoveTransaction endpoint.

The RemoveTransaction endpoint removes the transaction with the
provided txid including all its descendants from the internal wallet.

We still keep watching for the address of the transation in case
the transcation is confirmed nonetheless. This command is particular
useful for neutrino backends because new bitcoind versions do not
reply with an invalid transaction error code when the tx published
fails to be included into the mempool (fullnodes do).

lnrpc/walletrpc/walletkit.proto

@@ -208,6 +208,13 @@ service WalletKit {
     */
     rpc PublishTransaction (Transaction) returns (PublishResponse);
 
+    /* lncli: `wallet removetx`
+    RemoveTransaction attempts to remove the provided transaction from the
+    internal transaction store of the wallet.
+    */
+    rpc RemoveTransaction (GetTransactionRequest)
+        returns (RemoveTransactionResponse);
+
     /*
     SendOutputs is similar to the existing sendmany call in Bitcoind, and
     allows the caller to create a transaction that sends to several outputs at
@@ -751,6 +758,7 @@ message Transaction {
     */
     string label = 2;
 }
+
 message PublishResponse {
     /*
     If blank, then no error occurred and the transaction was successfully
@@ -762,6 +770,11 @@ message PublishResponse {
     string publish_error = 1;
 }
 
+message RemoveTransactionResponse {
+    // The status of the remove transaction operation.
+    string status = 1;
+}
+
 message SendOutputsRequest {
     /*
     The number of satoshis per kilo weight that should be used when crafting