Remove NewSessionCmd::desiredCapabilities

I can't find a reference to this attribute in the spec. And I cant
remember why I added it.

Author: equalsraf

src/messages.rs

@@ -5,7 +5,6 @@ use serde::de::{Visitor, MapAccess};
 use serde::de::Error as DeError;
 use serde::ser::SerializeStruct;
 use serde_json::Value as JsonValue;
-use serde_json::map::Map;
 use std::fmt;
 
 #[derive(Debug)]
@@ -37,7 +36,6 @@ pub struct WebDriverError {
 #[derive(Serialize)]
 pub struct NewSessionCmd {
     capabilities: JsonValue,
-    desiredCapabilities: JsonValue,
 }
 
 impl NewSessionCmd {
@@ -51,7 +49,6 @@ impl NewSessionCmd {
                     }
                 }
             }),
-            desiredCapabilities: JsonValue::Object(Map::new()),
         }
     }