Redirect subdomain www.sub.example.com to sub.example.com

I've been trying to do this for 2 days but to no avail.

In this directory(/etc/nginx/sites-available/sub.example.com) I added this line:

if ($host = www.sub.example.com) {
return 301 https://sub.example.com$request_uri;
}

In this other directory(/etc/nginx/nginx.conf) I added this line:
include /etc/nginx/sites-available/*;

I tested it with (nginx -t) and found no errors! but it does not redirect in any way.

Edit the site wo site edit sub.example.com

Don't use an 'if' for this ('if' is often a bad idea).

Add a server block for the www:-

server {
    server_name www.sub.example.com;
    return 301 https://sub.example.com$request_uri;
}

And remove www.sub.example.com from server_name in the main server block.