removed proxy arg group

[yas99en]

I found a bug.

$ sudo ./target/debug/doh-client -l 0.0.0.0:53  -d dns.google -r dns.google:443 --proxy-host 192.168.1.108:3128 --proxy-scheme http /etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem
error: The argument '--proxy-scheme <proxy-scheme>' cannot be used with one or more of the other specified arguments

I think the arguments group 'proxy' makes this error.