Those buttons are there because you installed an addon which puts them there. If you have no idea which addon might be doing it, you should (a) pay more attention to what addons you're installing, and (b) follow these steps to figure out what it is:
1. Type "/fstack" and press enter. You should now see a tooltip-like window in one corner of your screen, listing all of the frame objects that are underneath the mouse cursor.
2. Move the mouse cursor over the quest buttons, and look at the frame list tooltip you brought up #1. You should see something like "SomeAddonButton1" or "SomeAddonQuestBar" near the top of the list.
3. If you can identify the addon, type "/fstack" again to hide the frame list tooltip, and either disable that addon or, if you want to keep it but don't want the buttons in the middle of the screen, refer to its download page for instructions on how to move the buttons and access other configuration options.
4. If you cannot identify the addon, post a screenshot of the frame list tooltip while the mouse cursor is over the quest item buttons, and/or a list of addons you have enabled.
__________________ Retired author of too many addons.
Message me if you're interested in taking over one of my addons. Donít message me about addon bugs or programming questions.