fix(postal): remove invalid minecraft:icon block component

Bedrock 1.21+ rejects minecraft:icon as a *block* component (it's an
*item* component), failing block registration with "child 'minecraft:icon'
not valid here". The post_office and mailbox blocks never registered, so
their recipes failed (silverlabs:post_office / silverlabs:mailbox missing)
and the items didn't appear in the in-game inventory despite the addon
being marked active in the world settings.

Reverts the additions made in f126eeb to those two block JSONs only;
the rest of the multi-mailbox script logic from that commit is unchanged.
Inventory icons now fall back to the material_instances texture (same
visual as before f126eeb).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

postal-service-addon/postal_service_BP/blocks/mailbox.json

@@ -21,8 +21,7 @@
           "texture": "mailbox",
           "render_method": "opaque"
         }
-      },
-      "minecraft:icon": "mailbox_icon"
+      }
     }
   }
 }