mempool,rpc: add removetx rpc method (#7047)

Addresses one of the concerns with #7041.

Provides a mechanism (via the RPC interface) to delete a single transaction, described by its hash, from the mempool. The method returns an error if the transaction cannot be found. Once the transaction is removed it remains in the cache and cannot be resubmitted until the cache is cleared or it expires from the cache.

rpc/openapi/openapi.yaml

@@ -237,6 +237,31 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorResponse"
+
+  /remove_tx:
+    get:
+      summary: Removes a transaction from the mempool.
+      tags:
+        - TxKey
+      operationId: remove_tx
+      parameters:
+        - in: query
+          name: txKey
+          required: true
+          schema:
+            type: string
+            example: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+          description: The transaction key
+      responses:
+        "200":
+          description: empty response.
+        "500":
+          description: empty error.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorResponse"
+
   /subscribe:
     get:
       summary: Subscribe for events via WebSocket.