Remove unused and obsolete file from common
Remove unused and obsolete file from common

Remove section_about-dashboard.xml: Describes setup of horizon,
not needed since Install Guide covers it.

Backport: none

Change-Id: I15fc0a55507edd128ce8feecf9263ac48079f83e

AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,Akasurde/openstack-manuals,gongysh2004/openstack-manual,saeki-masaki/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,saeki-masaki/openstack-manuals,potsmaster/openstack-manuals