Remove incorrect geos dependency

- geos actually is no longer a dependency of Mapnik as of Mapnik v2.2.0 as the optional plugin was removed before the v2.0.0 release: https://github.com/mapnik/mapnik/commit/4c38bf9b81054730fc030c7ee88f1b4695e72542#diff-3e99938ea9939d25f744dede336de391