Don't drop the public schema when initializing the database

The production database has a public schema.  I thought we had dropped it, but
apparently not.  We should match what exists in prod either way.

(imported from commit 1bf956360029ebbd59afc3cc30fca9a859343adf)

tools/postgres-init-db

@@ -50,7 +50,6 @@ CREATE SCHEMA zulip;
 EOF
 
 $ROOT_POSTGRES $DBNAME_BASE << EOF
-DROP SCHEMA public CASCADE;
 CREATE EXTENSION tsearch_extras SCHEMA zulip;
 EOF