Bug 25353: Correct eslint errors in additems.js

Bug 25353: Correct eslint errors in additems.js

This patch makes minor changes to additem.js to correct errors
highlighted by eslint:

- Consistent indentation
- Notation of global and exported variables and functions
- Consistent use of the "var" declaration for all new variables
- Remove variables from function calls which aren't used

To test, apply the patch and use your preferred eslint-enabled tool to
check for errors. There should be none.

If you don't have eslint integrated in your development environment you
could use the online ESLint demo at https://eslint.org/demo.

The demo isn't configured to assume a browser environment with jQuery
present, so you'll have to modify the top line after pasting the
contents of additem.js:

/* global $ alert window __ */

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>