Remove unneeded help about identity servers

The custom server path no longer shows an identity server field (for modern
homeservers), so it's confusing to have the help dialog reference it.

Part of https://github.com/vector-im/riot-web/issues/11236

src/components/views/auth/CustomServerDialog.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2019 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -34,11 +35,6 @@ module.exports = createReactClass({
                         "allows you to use this app with an existing Matrix account on a " +
                         "different homeserver.",
                     )}</p>
-                    <p>{_t(
-                        "You can also set a custom identity server, but you won't be " +
-                        "able to invite users by email address, or be invited by email " +
-                        "address yourself.",
-                    )}</p>
                 </div>
                 <div className="mx_Dialog_buttons">
                     <button onClick={this.props.onFinished} autoFocus={true}>