Remove database section from plugin.ini

Change https://review.openstack.org/#/c/34515 moved the database
config items to neutron.conf but left some commented out
[database] items. Remove them as well.

Closes-Bug: #1247589
Change-Id: I05913d0c27a683cec30cae31a1978612aa1e1360
Merge "Remove database section from plugin.ini"