color: Fix GTK4 invisible start calibration button

The start button for the color calibration UI is missing. This has been a Launchpad Ubuntu bug for a while.

The attribute internal-child is invalid in GTK4, so the row with the start button doesn't load. This makes the calibration feature impossible to use. Removing the unused unrecognized attribute allows the start button to load again. Its value "action_area" was unused anyway.

The AskUbuntu asker confirmed that 'After implementing the patch from the answer by Daniel T (thanks BTW), it does now get a "Start" button, but after clicking it, the color patch remains white, and it errors out with "internal error".'

Before

After