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.swagger.json

@@ -506,6 +506,39 @@
         ]
       }
     },
+    "/v2/wallet/removetx": {
+      "post": {
+        "summary": "lncli: `wallet removetx`\nRemoveTransaction attempts to remove the provided transaction from the\ninternal transaction store of the wallet.",
+        "operationId": "WalletKit_RemoveTransaction",
+        "responses": {
+          "200": {
+            "description": "A successful response.",
+            "schema": {
+              "$ref": "#/definitions/walletrpcRemoveTransactionResponse"
+            }
+          },
+          "default": {
+            "description": "An unexpected error response.",
+            "schema": {
+              "$ref": "#/definitions/rpcStatus"
+            }
+          }
+        },
+        "parameters": [
+          {
+            "name": "body",
+            "in": "body",
+            "required": true,
+            "schema": {
+              "$ref": "#/definitions/walletrpcGetTransactionRequest"
+            }
+          }
+        ],
+        "tags": [
+          "WalletKit"
+        ]
+      }
+    },
     "/v2/wallet/reserve": {
       "get": {
         "summary": "lncli: `wallet requiredreserve`\nRequiredReserve returns the minimum amount of satoshis that should be kept\nin the wallet in order to fee bump anchor channels if necessary. The value\nscales with the number of public anchor channels but is capped at a maximum.",
@@ -1432,6 +1465,15 @@
         }
       }
     },
+    "walletrpcGetTransactionRequest": {
+      "type": "object",
+      "properties": {
+        "txid": {
+          "type": "string",
+          "description": "The txid of the transaction."
+        }
+      }
+    },
     "walletrpcImportAccountRequest": {
       "type": "object",
       "properties": {
@@ -1775,6 +1817,15 @@
     "walletrpcReleaseOutputResponse": {
       "type": "object"
     },
+    "walletrpcRemoveTransactionResponse": {
+      "type": "object",
+      "properties": {
+        "status": {
+          "type": "string",
+          "description": "The status of the remove transaction operation."
+        }
+      }
+    },
     "walletrpcRequiredReserveResponse": {
       "type": "object",
       "properties": {