Remove invalid attribute from idle AI

It was interpreted as a simplified aspect, resulting in a warning when
loading a replay.

Found with WML unit tests.

data/ai/ais/idle_ai.cfg

@@ -2,7 +2,6 @@
 [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]