How to remove box outlines around parenthesis/brackets in TexStudio?

I found the answer myself. These boxes are called placeholders. They can be removed by going to Options > Configure TexStudio. Then on the bottom left check Show Advanced Options. Go to the Completion tab on the left and in this tab uncheck Arguments as Placeholders on the top right. See the attached image.

The boxes are placeholders. You can navigate between them using Ctrl+Left/Right.

They vanish, if you type over them e.g. if the cursor is before a closing bracket placeholder ), typing ) will move the cursor past and remove the placeholder.

You can also use Idefix -> Remove Placeholder (Ctrl+Shift+K).