Revert "add prefix index on tags (#3858)"

This reverts commit 8c564f055f.

app/logical/tag_autocomplete.rb

@@ -74,9 +74,9 @@ module TagAutocomplete
       n += 2
     end
 
-    regexp = "([a-z0-9])[a-z0-9']*($|[^a-z0-9']+)"
+    anchors = "^" + query.split("").map {|x| Regexp.escape(x)}.join(".*[#{PREFIX_BOUNDARIES}]")
     Tag
-      .where('regexp_replace(name, ?, ?, ?) like ?', regexp, '\1', 'g', query.to_escaped_for_sql_like + '%')
+      .where("name ~ ?", anchors)
       .where("post_count > ?", min_post_count)
       .where("post_count > 0")
       .order("post_count desc")

db/structure.sql

@@ -7128,13 +7128,6 @@ CREATE UNIQUE INDEX index_tags_on_name ON public.tags USING btree (name);
 CREATE INDEX index_tags_on_name_pattern ON public.tags USING btree (name text_pattern_ops);
 
 
---
--- Name: index_tags_on_name_prefix; Type: INDEX; Schema: public; Owner: -
---
-
-CREATE INDEX index_tags_on_name_prefix ON public.tags USING gin (regexp_replace((name)::text, '([a-z0-9])[a-z0-9'']*($|[^a-z0-9'']+)'::text, '
'::text, 'g'::text) public.gin_trgm_ops) WHERE (post_count > 0);
-
-
 --
 -- Name: index_tags_on_name_trgm; Type: INDEX; Schema: public; Owner: -
 --
@@ -7531,7 +7524,6 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20180517190048'),
 ('20180518175154'),
 ('20180804203201'),
-('20180816230604'),
+('20180816230604');
-('20180912185624');