Enable MD5 check to be disabled

Some data repositories it seems do not have an MD5 file associated with the download:
https://github.com/dartov/nominatim-install/commit/eebed641d83efcac303e27320a90bf66ea251dbb
https://github.com/xivk/nominatim-install/commit/b2664beee4982e51e173d2f7f284e8bb6bc1dc43

Enable the check to be disabled.
