Just wipe the eq2.ini file from your base EQ2 directory, as this is the file that directs the game to load a desired ui mod. You may as well also delete the custom ui folder from your EverQuest II\UI directory, since you're not gonna be using it (you can leave it there as long as you delete the eq2.ini file, but why?
).
Then the next time you load into the game, it will load the default UI (the game will recreate the eq2.ini file with default info).