chg: [retention] hide_tag removed to validate current schema

Maybe we could improve the format to include it by default to
taxonomy format to trigger the MISP hide tag functionality directly.

{'value': 'expired', 'expanded': 'Set when the retention period has expired', 'numerical_value': 0, 'hide_tag': True}: Additional properties are not allowed ('hide_tag' was unexpected)

retention/machinetag.json

@@ -10,8 +10,7 @@
     {
       "value": "expired",
       "expanded": "Set when the retention period has expired",
-      "numerical_value": 0,
-      "hide_tag": true
+      "numerical_value": 0
     },
     {
       "value": "1d",