This plugin removes the Categories list from the Items menu and shows all items in a single window. Key items are included, but not Hidden items.
How to use
Add to your plugins folder (file name does not matter, but I call mine NoItemCategories.js). There are no parameters to set or plugin commands to call.
This plugin overwrites a number of functions from Scene_Shop and Scene_Item. It will likely have compatibility issues with other plugins that modify the items and shop menus. I will not provide fixes/patches for these incompatibilities, though you are free to make them yourself or request someone else to make them.