Easiest thing to do is delete the default.cfg file in the cfg folder, but you'll lose all the other remaps saved in that file (everything you remapped using "Input (general)" menu).
Or you can open that file in a text editor, and search for "
UI_CONFIGURE". Delete that
<port...>... to the next
</port>, or just look and see what it's changed to.
Also, this is a
mame FAQ.