Delete downloaded zip file after unpacking it.

[baronlanteigne]

I notice LilySurfaceScraper uses more disk space than it should by not deleting the downloaded zip file after unpacking it. Only the image files are used so there is no point to leave the zip file there.

[Elie Michel]

Thanks for the suggestion, will be fixed in the next release (the zip file will still be present but as a 0-byte file, to let the add-on know that the maps do not need to be downloaded again).

(edit: I actually updated the v1.7.4 directly with this feature)