Avoid double restarts of the query server

Previously, if there was an explicit restart of the query server (eg-
by changing a configuration setting), then the query server process
would be started twice: once by the `close` handler and once by the
restart command.

By adding the `removeAllListeners` to the dispose method, we ensure that
when the query server shuts down gracefully, there won't be a `close`
listener that is going to restart it a second time if there is a
different way of restarting it.

extensions/ql-vscode/src/query-server/server-process.ts

@@ -26,6 +26,7 @@ export class ServerProcess implements Disposable {
     this.connection.end();
     this.child.stdin!.end();
     this.child.stderr!.destroy();
+    this.child.removeAllListeners();
     // TODO kill the process if it doesn't terminate after a certain time limit.
 
     // On Windows, we usually have to terminate the process before closing its stdout.