Description of problem:
If you had set a proxy username and password in the advance network configuration dialog in firstboot, you cannot unset it by unchecking the option as you can in subscription-manager-gui.

Version-Release number of selected component (if applicable):
subscription-manager-firstboot-0.96.1-1.git.186.1107890.el6.x86_64

How reproducible:
always

Steps to Reproduce:
1. Fire up firstboot
2. Go through the menus until you get to register updates
3. Choose to register with subscription manager
4. Set your proxies by going to 'advanced network configuration'
 - pick a proxy server that allows you to authenticate with a user name and password
5. Hit forward, and register subscription manager.
6. Change your mind about the proxy.
7. Hit the back button until you get to the advanced network configuration again.
8. Pick a proxy server that does not allow authentication and choose not to use a proxy username and password by UNCHECKING the "use username and password" box.
9. Go forward and register again, completing firstboot.

  
Actual results:
In your /etc/rhsm/rhsm.conf file, you will notice that your proxy_username and proxy_password are still set to the old values.

Expected results:
The two values have been blanked as you opted not to use them with the checkbox. (just like subscription-manager-gui).