To redirect a www domain to the « root » domain with Nginx, one always prefers not to make any use of the if syntax (because it's evil), and rather use two different server directives for each rule :

server {
    server_name www.domain.com;
    return 301 $scheme://domain.com$request_uri; 
}
server {
    server_name domain.com;	
    # Much more cool stuff here
}

Of course a 301 is a permanent redirection

Then :

sudo service nginx restart

Obviously, we don't want to rekindle discussions over whether it's a good practice or not(see this article or these bunches of pro, cons and WTF).