Hello,
Menu dropdowns are created if you have a multi-levels menu. If you don’t want dropdowns, just make sure you don’t add multiple levels to your menu.
You can control your menu structure from Appearance > Menus (see https://codex.wordpress.org/WordPress_Menu_User_Guide for extensive instructions).
Note that if you don’t create your own menu as outlined in this guide, then WordPress will automatically create a menu for you, but you won’t have any control over its structure.
