fix(layer aliases): Remove duplicate continent entry

For some time, apparently, there have been two `continent` entries in
the `layer_alias` default configuration. Overall, this hasn't had any
significant negative effect, since this list is mostly used in places
that eventually deduplicate it.

Worst case, some Elasticsearch filters and error messages have listed
the continent layer twice, which is redundant but not going to break
anything.

It's obviously better fixed though, and makes writing some API unit
tests a bit simpler :)

config/defaults.json

@@ -50,7 +50,7 @@
         "whosonfirst": [
           "continent", "empire", "country", "dependency", "macroregion", "region", "locality",
           "localadmin", "macrocounty", "county", "macrohood", "borough", "neighbourhood",
-          "microhood", "disputed", "venue", "postalcode", "continent", "ocean", "marinearea"
+          "microhood", "disputed", "venue", "postalcode", "ocean", "marinearea"
         ]
       },
       "source_aliases": {
@@ -63,7 +63,7 @@
         "coarse": [
           "continent", "empire", "country", "dependency", "macroregion", "region", "locality",
           "localadmin", "macrocounty", "county", "macrohood", "borough", "neighbourhood",
-          "microhood", "disputed", "postalcode", "continent", "ocean", "marinearea"
+          "microhood", "disputed", "postalcode", "ocean", "marinearea"
         ]
       }
     }

test/expected-deep.json

@@ -55,7 +55,7 @@
         "whosonfirst": [
           "continent", "empire", "country", "dependency", "macroregion", "region", "locality",
           "localadmin", "macrocounty", "county", "macrohood", "borough", "neighbourhood",
-          "microhood", "disputed", "venue", "postalcode", "continent", "ocean", "marinearea"
+          "microhood", "disputed", "venue", "postalcode", "ocean", "marinearea"
         ]
       },
       "source_aliases": {
@@ -68,7 +68,7 @@
         "coarse": [
           "continent", "empire", "country", "dependency", "macroregion", "region", "locality",
           "localadmin", "macrocounty", "county", "macrohood", "borough", "neighbourhood",
-          "microhood", "disputed", "postalcode", "continent", "ocean", "marinearea"
+          "microhood", "disputed", "postalcode", "ocean", "marinearea"
         ]
       }
     }