Refactor deprecated App::getArgumentValue() to DI::args()->get()

src/App.php

@@ -543,16 +543,6 @@ class App
 		return Core\Theme::getStylesheetPath($this->getCurrentTheme());
 	}
 
-	/**
-	 * @deprecated use Arguments->get() instead
-	 *
-	 * @see        App\Arguments
-	 */
-	public function getArgumentValue($position, $default = '')
-	{
-		return $this->args->get($position, $default);
-	}
-
 	/**
 	 * Sets the base url for use in cmdline programs which don't have
 	 * $_SERVER variables