Remove getScreenshot method from FirefoxDriver as it's in RemoteWebDriver

Try and reduce the amount of code we have to maintain.

Author: shs96c

java/client/src/org/openqa/selenium/firefox/FirefoxDriver.java

@@ -26,7 +26,6 @@
 
 import org.openqa.selenium.Capabilities;
 import org.openqa.selenium.NoSuchSessionException;
-import org.openqa.selenium.OutputType;
 import org.openqa.selenium.Proxy;
 import org.openqa.selenium.WebDriverException;
 import org.openqa.selenium.firefox.internal.NewProfileExtensionConnection;
@@ -417,13 +416,6 @@ private static Capabilities dropCapabilities(Capabilities capabilities) {
     return caps;
   }
 
-  public <X> X getScreenshotAs(OutputType<X> target) {
-    // Get the screenshot as base64.
-    String base64 = execute(DriverCommand.SCREENSHOT).getValue().toString();
-    // ... and convert it.
-    return target.convertFromBase64Png(base64);
-  }
-
   public static class LazyCommandExecutor implements CommandExecutor, NeedsLocalLogs {
     private ExtensionConnection connection;
     private final FirefoxBinary binary;