remove ubunut 18.04 from prod notes until 4.0.1

Author: kay-kim

source/administration/production-notes.txt

@@ -73,7 +73,6 @@ systems are recommended for production use:
 - RHEL / CentOS 6.2+
 - SLES 12
 - Ubuntu LTS 16.04
-- Ubuntu LTS 18.04
 - Windows Server 2012 R2
 
 .. seealso:: :ref:`prod-notes-platform-considerations`