Ruby: remove unused predicate

ruby/ql/lib/codeql/ruby/frameworks/ActionDispatch.qll

@@ -845,14 +845,6 @@ module ActionDispatch {
         )
   }
 
-  /**
-   * Convert the first character of the string to lowercase.
-   */
-  bindingset[input]
-  private string decapitalize(string input) {
-    result = input.charAt(0).toLowerCase() + input.suffix(1)
-  }
-
   /**
    * Strip leading and trailing forward slashes from the string.
    */