Only append 'Elastic/Synthetics' to default UA

When we originally implemented
elastic#232 it appears we didn't
implement the third AC, allowing users to override our custom UA string
"Elastic/Synthetics" which is appended.

Some users need to fully replace the UA, this allows them to do that.
Rather than adding a boolean, this assumes that if you're overriding the
UA you want full control, which is more in line with the principle of
least surprise.

src/core/gatherer.ts

@@ -65,12 +65,12 @@ export class Gatherer {
   }
 
   static async getUserAgent(userAgent?: string) {
-    const syntheticsIdentifier = ' Elastic/Synthetics';
     if (!userAgent) {
       const session = await Gatherer.browser.newBrowserCDPSession();
       ({ userAgent } = await session.send('Browser.getVersion'));
+      return userAgent + ' Elastic/Synthetics';
     }
-    return userAgent + syntheticsIdentifier;
+    return userAgent;
   }
 
   static setNetworkConditions(