Removing legacy WebDriver extension when using Geckodriver in .NET

When a `FirefoxProfile` object is created in the .NET bindings, the legacy
WebDriver extension is included in that profile by default. This includes
the case where the user is using Geckodriver. When using Geckodriver, the
legacy extension is doing nothing more than taking up space. This commit
adds an internal method to detect when a `FirefoxProfile` object is being
used with geckodriver, and removes the extension from the list of
extensions in the profile before the profile is serialized to disk, and
subsequently to a base64-encoded string for transmission over the wire to
geckodriver. Fixes issue SeleniumHQ#6043.

dotnet/src/webdriver/Firefox/FirefoxOptions.cs

@@ -354,6 +354,9 @@ public override ICapabilities ToCapabilities()
 
             if (this.profile != null)
             {
+                // Using Marionette/Geckodriver, so the legacy WebDriver extension
+                // is not required.
+                this.profile.RemoveWebDriverExtension();
                 firefoxOptions[FirefoxProfileCapability] = this.profile.ToBase64String();
             }
 

dotnet/src/webdriver/Firefox/FirefoxProfile.cs

@@ -327,6 +327,18 @@ internal void AddWebDriverExtension()
             }
         }
 
+        /// <summary>
+        /// Removes the WebDriver extension for Firefox to the profile, for use with non-legacy
+        /// FirefoxDriver instances that use geckodriver.
+        /// </summary>
+        internal void RemoveWebDriverExtension()
+        {
+            if (this.extensions.ContainsKey("webdriver"))
+            {
+                this.extensions.Remove("webdriver");
+            }
+        }
+
         /// <summary>
         /// Internal implementation to set proxy preferences for this profile.
         /// </summary>