That's just an area set for in/out. So ... remove the in and out point, it's gone.
Shift-i makes an in point. Shift-o makes an out point.
Add Ctrl (Cmd) to either one clears that point ... Ctrl-Shift-i clears the in-point, for example.
There are many keyboard shortcuts you need to study and learn to both speed up and refine your editing ability.
As Neil said, you just have In and Out Marks set. Press Alt (WIN) or Option (macOS) + X to Clear In and Out Marks.