[world conquest] World Conquest Era is only available for the WC campaign

Until a proper solution is found for #5228 , this will have to do. It suppresses the appearance of the era in the era selection list for other scenarios.
Hmm, I guess the [campaign] tag should support the allow_era= key in future but for the moment, this will do.

data/campaigns/World_Conquest/era/era.cfg

@@ -8,6 +8,10 @@
         id= "{STR_ERA_ID_WC_II}"
         name= {STR_ERA_NAME_WC_II}
         description= {STR_ERA_DESCRIPTION_WC_II}
+
+        allow_scenario=WC_II_1p,WC_II_2p,WC_II_3p
+        hide_help=yes
+
         require_era=no
         # addon_min_version="8.2"