In initdb's post-bootstrap phase, drop temp tables explicitly.

Although these temp tables will get removed from template1 at the end of
the standalone-backend run, that's too late to keep them from getting
copied into the template0 and postgres databases, now that we use only a
single backend run for the whole sequence.  While no real harm is done
by the extra copies (since they'd be deleted on first use of the temp
schema), it's still unsightly, and it would mean some wasted cycles for
every database creation for the life of the installation.

Oversight in commit c4a8812c.  Noticed by Amit Langote.