icons: Remove ZedAssistant2 icon (#27634)

This PR removes the `ZedAssistant2` icon.

I went to use it as a placeholder icon, but noticed that the icon wasn't
loaded properly due to a name mismatch.

However, since we aren't using it anywhere I'm opting to remove it.

Release Notes:

- N/A

crates/icons/src/icons.rs

@@ -231,7 +231,6 @@ pub enum IconName {
     X,
     XCircle,
     ZedAssistant,
-    ZedAssistant2,
     ZedAssistantFilled,
     ZedPredict,
     ZedPredictUp,