Better meta-schema for hrefPointers

Specify the actual object and property types, and drop
the description as we no longer keep those in the meta-schema.

Author: handrews

links.json

@@ -10,7 +10,10 @@
             "format": "uri-template"
         },
         "hrefPointers": {
-            "description": "a map of variable names to relative instance JSON Pointers, which adjust the instance location from which template variable resolution begins"
+            "type": "object",
+            "additionalProperties": {
+              "type": "string"
+            }
         },
         "hrefSchema": {
             "allOf": [