'jekins:label' not w3c compatible when passed to endpoint node. (IEDriverServer)

Discovered because I've had an issue starting Internet Explorer on specific nodes passing 'jenkins:label' to the Selenium Plugin grid.
```
Exception in thread "main" org.openqa.selenium.SessionNotCreatedException: No matching capability sets found.
Unable to match capability set 0: jenkins:label is an unknown extension capability for IE
```

The w3c specification say for [New Session](https://w3c.github.io/webdriver/#dfn-new-sessions) says:
> If the remote end is an intermediary node, it may use the result of the capabilities processing algorithm to route the new session request to the appropriate endpoint node.
> An intermediary node is free to define extension capabilities to assist in this process, however, these specific capabilities *must not be forwarded to the endpoint node.*

:warning: Fixing this properly will depend on feature request with selenium grid to [add configuration for custom capabilities mutators #7478](https://github.com/SeleniumHQ/selenium/issues/7478)