pgroonga: Remove now-unnecessary 'GRANT USAGE' statement.

This was only necessary for PGroonga 1.x, and the `pgroonga` schema
will most likely be removed at some point inthe future, which will
make this statement error out.

Drop the unnecessary statement.

(cherry picked from commit dc2726c814)

scripts/setup/create-pgroonga.sql

@@ -1,3 +1,2 @@
 \connect zulip
 CREATE EXTENSION pgroonga;
-GRANT USAGE ON SCHEMA pgroonga TO zulip;