Bug 1487999: webdriver: simplify cloning of merged_capabilities; r=whimboo

We clone merged_capabilities in the CapabilitiesMatching implementation
for SpecNewSessionParameters::match_browser using x.map(|x| x.clone()),
but this can be simplified to x.cloned().

testing/webdriver/src/capabilities.rs

@@ -510,7 +510,7 @@ impl CapabilitiesMatching for SpecNewSessionParameters {
                 Some(merged)
             })
             .next()
-            .map(|x| x.clone());
+            .cloned();
         Ok(selected)
     }
 }