Mas o funciona . A resposta correta parece já estar na sua pergunta.
$ cat in.log
http://example.mx
https://test.com
http://4.3.4.4
http://dev.somedomain.com
http://1.3.4.2
$ cat in.log | sed '/^http:\/\/[0-9]/d' > out.log
$ cat out.log
http://example.mx
https://test.com
http://dev.somedomain.com
$