Remove dependency on deprecated XQueryServlet

[joewiz]

As per https://github.com/eXist-db/exist/issues/555 and discussed during today's Community Call, XQueryServlet is deprecated and may be removed as soon as eXist 6.

XQueryServlet is used in eXide's execution of XQuery submitted by the client. See https://github.com/eXist-db/eXide/blob/develop/controller.xql#L164-L223.

As an alternative, we should switch to the method of executing XQuery used in the Atom plugin, VSCode plugin, and Fusion Studio. See links in https://github.com/eXist-db/eXide/pull/284.