fairland: Drop option -a

fairland: Drop option -a

With -a, fairland makes the capital sector an airfield to "mark the
continents [...] so that you can tell them from the islands".  This is
pretty redundant since commit afc0ef94e "Make fairland record the
island number in the deity territory", v4.3.31.  Drop it.

The map fairland prints is not affected.  The continents are clearly
visible there.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>