Bug 1477475 - Drop legacy Marionette capabilities duplication. r=whimboo

geckodriver currently writes a legacy capabilities structure to the
WebDriver:NewSession command it uses on creating a new WebDriver
session which duplicates the top-level object:

	{capabilities: {<caps>, desiredCapabilities: <caps>}}

Where caps can look something like:

	{acceptInsecureCerts: true, pageLoadStrategy: "normal", ...}

Marionette has picked up the top-level "capabilities" key for a
very long time now, and there is no longer any need to duplicate
the body.

testing/geckodriver/src/marionette.rs

@@ -1130,11 +1130,6 @@ impl MarionetteCommand {
                     data.insert(k.to_string(), v.to_json());
                 }
 
-                // duplicate in capabilities.desiredCapabilities for legacy compat
-                let mut legacy_caps = BTreeMap::new();
-                legacy_caps.insert("desiredCapabilities".to_string(), caps.to_json());
-                data.insert("capabilities".to_string(), legacy_caps.to_json());
-
                 (Some("WebDriver:NewSession"), Some(Ok(data)))
             }
             PerformActions(ref x) => (Some("WebDriver:PerformActions"), Some(x.to_marionette())),