Remove readonly attribute from identities collection in users (#4442)

Author: solankisamir

specification/apimanagement/resource-manager/Microsoft.ApiManagement/preview/2018-06-01-preview/apimusers.json

@@ -822,7 +822,6 @@
           "description": "Optional note about a user set by the administrator."
         },
         "identities": {
-          "readOnly": true,
           "type": "array",
           "items": {
             "$ref": "#/definitions/UserIdentityContract"

specification/apimanagement/resource-manager/Microsoft.ApiManagement/stable/2018-01-01/apimusers.json

@@ -822,7 +822,6 @@
           "description": "Optional note about a user set by the administrator."
         },
         "identities": {
-          "readOnly": true,
           "type": "array",
           "items": {
             "$ref": "#/definitions/UserIdentityContract"