Remove SERVER env var in integration test.
Remove SERVER env var in integration test.

Per @Kuba, this is no longer needed for the letsencrypt client.

patf/boulder,ibukanov/boulder,jfrazelle/boulder,sophie-h/boulder,benileo/boulder,ZCloud-Firstserver/boulder,r0ro/boulder,mozmark/boulder,jsha/boulder,patf/boulder,mehmooda/boulder,jsha/boulder,sophie-h/boulder,ZCloud-Firstserver/boulder,mehmooda/boulder,jcjones/boulder,jcjones/boulder,jsha/boulder,hlandauf/boulder,jcjones/boulder,benileo/boulder,r0ro/boulder,r0ro/boulder,benileo/boulder,lucasallan/boulder,mehmooda/boulder,jfrazelle/boulder,letsencrypt/boulder,postfix/boulder,jcjones/boulder,patf/boulder,mozmark/boulder,letsencrypt/boulder,deserted/boulder,KyleChamberlin/boulder,mozmark/boulder,benileo/boulder,postfix/boulder,bmw/boulder,patf/boulder,mehmooda/boulder,lucasallan/boulder,bmw/boulder,ZCloud-Firstserver/boulder,ZCloud-Firstserver/boulder,hlandauf/boulder,postfix/boulder,KyleChamberlin/boulder,hlandauf/boulder,ibukanov/boulder,hlandauf/boulder,deserted/boulder,ibukanov/boulder,bmw/boulder,r0ro/boulder,lucasallan/boulder,deserted/boulder,deserted/boulder,letsencrypt/boulder,KyleChamberlin/boulder,jfrazelle/boulder,KyleChamberlin/boulder,postfix/boulder,jfrazelle/boulder,sophie-h/boulder,mozmark/boulder