Remove obsolete http_username, http_password from skel/sample.conf

[jvanasco]

These lines in the config file seem wrong

https://github.com/Supervisor/supervisor/blob/master/supervisor/skel/sample.conf#L52-L53

[supervisorctl]
...
;username=chris              ; should be same as http_username if set
;password=123                ; should be same as http_password if set

I think they are supposed to read

[supervisorctl]
...
;username= user              ; should be same as username if set in unix_http_server or inet_http_server
;password=123                ; should be same as password if set in unix_http_server or inet_http_server

[mnaberez]

Yes, it looks like we missed updating these comments. http_username and http_password are old options that were removed in Supervisor 3.