Update _x11_config.py to remove packagegroup-ni-graphical

[tkrebes]

Summary of Changes

Remove the packagegroup-ni-graphical package before removing packagegroup-ni-xfce.

Justification

When trying to remove packagegroup-ni-xfce, nilrt-snac configure fails with the following error:

 * Solver encountered 1 problem(s):
 * Problem 1/1:
 *   - package packagegroup-ni-graphical-1.0-r0.174.x64 requires packagegroup-ni-xfce, but none of the providers can be installed
 *   - conflicting requests
 *   - problem with installed package packagegroup-ni-graphical-1.0-r0.174.x64
 *
 * Solution 1:
 *   - allow deinstallation of packagegroup-ni-graphical-1.0-r0.174.x64

 * Solution 2:
 *   - do not ask to deinstall packagegroup-ni-xfce

 *   - do not ask to deinstall packagegroup-ni-xfce

Testing

On a nilrt VM,

• Ran nilrt-snac configure --yes
• Rebooted target
• Ran nilrt-snac verify

All operations completed succefully.

Procedure

• This PR: changes user-visible behavior, fixes a bug, or impacts the project's security profile; and so it includes a CHANGELOG note.
• I certify that the contents of this pull request complies with the Developer Certificate of Origin.