search: Remove now unnecessary tsearch_extra dependency.

Now that we're implemented tsearch_extras in pure postgres, we no
longer need a custom extension.  This should help us considerably, as
it means we no longer need to ship custom apt packages at all.

Fixes #467.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

scripts/setup/create-db.sql

@@ -3,4 +3,3 @@ ALTER ROLE zulip SET search_path TO zulip,public;
 CREATE DATABASE zulip OWNER=zulip;
 \connect zulip
 CREATE SCHEMA zulip AUTHORIZATION zulip;
-CREATE EXTENSION tsearch_extras SCHEMA zulip;