[postgres] fix CREATE UNIQUE INDEX statement

IF NOT EXISTS makes sense only when you provide the name of the index.

devops/postgres/schema.sql

@@ -265,14 +265,14 @@ CREATE INDEX        ON public.nodes USING btree (user_id, typename, parent_id);
 CREATE INDEX        ON public.nodes USING btree (id, typename, date ASC);
 CREATE INDEX        ON public.nodes USING btree (id, typename, date DESC);
 CREATE INDEX        ON public.nodes USING btree (typename, id);
-CREATE UNIQUE INDEX IF NOT EXISTS ON public.nodes USING btree (hash_id);
+CREATE UNIQUE INDEX ON public.nodes USING btree (hash_id);
 
 CREATE INDEX        ON public.contexts USING gin (hyperdata);
 CREATE INDEX        ON public.contexts USING btree (user_id, typename, parent_id);
 CREATE INDEX        ON public.contexts USING btree (id, typename, date ASC);
 CREATE INDEX        ON public.contexts USING btree (id, typename, date DESC);
 CREATE INDEX        ON public.contexts USING btree (typename, id);
-CREATE UNIQUE INDEX IF NOT EXISTS ON public.contexts USING btree (hash_id);
+CREATE UNIQUE INDEX ON public.contexts USING btree (hash_id);
 
 CREATE INDEX ON public.nodescontexts_nodescontexts USING btree (nodescontexts1, nodescontexts2);
 -- CREATE UNIQUE INDEX ON public.nodes USING btree (((hyperdata ->> 'uniqId'::text)));
@@ -436,8 +436,3 @@ EXECUTE PROCEDURE check_node_stories_json();
   ON ngrams
   FOR EACH ROW
   EXECUTE PROCEDURE check_ngrams_json();
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