chore: remove dead hostnames

[hacdias]

Superseeds #276 and removes all domains (or subdomains) that are no longer registered. I manually checked them using some online tools afterwards and they are all indeed not registered anymore.

After this PR, we should consider automating this kind of checks.

@lidel I don't have permissions on this repo, so I can't ask for your review.

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[hacdias]

Script used:

const dns = require('dns')
const gateways = require('./src/gateways.json')

function hostnameExists(hostname) {
  return new Promise((resolve) => {
    dns.lookup(hostname, (error) => resolve(!error))
  })
}

(async () => {
  const filteredGateways = []

  for (const gateway of gateways) {
    const hostname = new URL(gateway).hostname
    const exists = await hostnameExists(hostname)

    if (exists) {
      filteredGateways.push(gateway)
    }
  }

  console.log(JSON.stringify(filteredGateways, null, "	"))
})()