[whatwg/fetch] Remove FTP support (#1009)

It seems Chrome and Firefox are both removing FTP support.

Potentially impacted parts of the spec:

- [Scheme fetch](https://fetch.spec.whatwg.org/#scheme-fetch)
- [Network scheme](https://fetch.spec.whatwg.org/#network-scheme): maybe this can be removed in favor of "HTTP(S) scheme"? Or rename "HTTP(S) scheme" to "network scheme"? Unsure what this impacts.
- [Bad port blocking](https://fetch.spec.whatwg.org/#port-blocking) (connected to network scheme)
- [Main fetch](https://fetch.spec.whatwg.org/#main-fetch) step 2.8 appears to be attempting to network-error some FTP requests. If we instead network-error all FTP requests then this step can probably be deleted.

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