Remove readOnly's description in validation meta-schema

The readOnly is property is actually the only one having a description;
it got inherited when moved from hyper-schema meta-schema. So drop it to
be consistent with the rest of the schema.

Additionnally, the current description contains ambiguous "server
language" which we try to avoid.

Author: dlax

schema.json

@@ -58,7 +58,6 @@
         },
         "default": {},
         "readOnly": {
-            "description": "If true, indicates that the value of this property is controlled by the server.",
             "type": "boolean",
             "default": false
         },