Remove admin_level=8 from the admin_city_names index

As we no longer restrict rendering to administrative boundaries of
level 8, rework the index for city name lookups.

NOTE: if you have an existing PostGIS database, you will need to
recreate the index. First, remove the existing one with:
`DROP INDEX admin_city_names` then re-create it with the
`CREATE INDEX ...` statement from ocitysmap-init.sql.

Signed-off-by: Maxime Petazzoni <maxime.petazzoni@bulix.org>

ocitysmap-init.sql

@@ -19,15 +19,14 @@
 -- You should have received a copy of the GNU Affero General Public License
 -- along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
--- Create a partial index to speed up the city name lookups (a few
--- milliseconds versus a few minutes):
-create index admin_city_names
+-- Create a partial index to speed up the city name/administrative boundaries
+-- lookups (a few milliseconds versus a few minutes):
+create index admin_boundaries_names
        on planet_osm_line (boundary,admin_level,name)
-       where (boundary='administrative' and admin_level='8');
+       where (boundary='administrative');
 
 -- Create an aggregate used to build the list of squares that each
 -- street intersects
-
 CREATE AGGREGATE textcat_all(
   basetype    = text,
   sfunc       = textcat,