Remove support for FTP proxy

[whimboo]

Browsers are going to drop FTP (proxy) support. As such we should consider to remove ftpProxy from the proxy capability.

For Firefox it will be removed in the upcoming 90.0 release. But not sure about Chrome and Safari.

@JohnChen0 and @gsnedders do you have some details?

[JohnChen0]

I don't know what Chrome's plan is. @foolip Do you have the info or know who to contact?

[gsnedders]

do you have some details?

Given SafariDriver doesn't support proxies at all, I'm pretty sure we're fine with support for ftpProxy being dropped from the spec.