You use the unnumber command to remove the COBOL line numbers from source code:
This removes any numbers and text that are in the area for the selected line numbering style.
For example, if you have configured the editor to use COBOL line numbering, unnumbering removes all text from the COBOL sequence area.
You can use Undo to revert the changes.
The style of the line numbers is specified in Window > Preferences > Micro Focus > COBOL > Editor > Line Numbering.