Can't you open up the MCP file again, unselect the map fix, and click "Apply Patches"? Don't know if that reverses the change or not.
That's basically what I did in order to mess it up, except I chose to "uninstall all", as I figured you can't just uninstall one fix.
Anyway, the Wrye Mash thing worked, sort of. Except now I have huge gridlines on my map that weren't there before and I've still got tiny little squares from the towns that got marked with the expanded map. I wonder if those will go away if I travel through those areas and overwrite them?