Remove dead seed addresses

[Saiv46]

Some seeds are either unavailable or using self-signed certificates, and this causes a bootstrap delay for Nimiq nodes (including Wallet). I'd recommend setting up some availability monitoring or something.

1. nimiq.icemining.ca:8443 at seeds.txt#L10

error code: 521* TLSv1.2 (IN), TLS header, Supplemental data (23):

2. seed.nimiqchina.com:8443 at seeds.txt#L12

Could not resolve host: seed.nimiqchina.com

3. seed.nimiq.jp:8443 at seeds.txt#L13

SSL certificate problem: self-signed certificate

4. nimiq.surf:8443 at seeds.txt#L14

502 Bad Gateway