Fixes various issues including flickering nav items, and smint-disabled items highlighting

Fixed: 
• Nav items flickering when scrolling past the bottom of the window
• Nav items of class smint-disable highlighting while scrolling despite not being a section of the page
• Uncaught Type Error thrown while scrolling

Also removed the jquery.smint.js from the main folder as it was not an exact duplicate of the jquery.smint.js in the demo/js folder. The main folder js file contained errors, while the demo js file works properly.