To remove a persona from a journey map simply hover over the persona panel on your CJM and hit X in the upper right corner.
This will not delete the persona, but only remove it from this particular CJM. You can then link this journey map to any other (or that same) persona from the project.
Note: By removing a persona from a map, you also remove all the persona's subsections from the map, and you won't be able to restore them.
If you want to keep persona-related information, copy it to the All Personas subsection or temporarily hide the persona that you don't need.
You can also replace the existing persona with another one:
Note: To make it work, you need to have at least 1 persona inside your project that is not added to the map.