Revert "Remove invalid attribute from idle AI"

This reverts commit 1d84d83.

I had introduced use of the hidden= key in b2df837, but
apparently didn't add them to the valid list of non-aspect keys.

data/ai/ais/idle_ai.cfg

@@ -2,6 +2,7 @@
 [ai]
     id=idle_ai
     description=_"Multiplayer_AI^Dev AI: Idle AI" # wmllint: no spellcheck
+    hidden=yes
     # Needs to define at least one stage; otherwise, the default AI will be injected
     # when used with [modify_side]switch_ai or when loading a mid-scenario save
     [stage]