delete dsh group "mediawiki-installation-precise"

nowadays everything is precise and there seems to be no use for it,
while "mediawiki-installation" is the important one for scapping.

most likely leftover from upgrading in the past?

Change-Id: I277056eb46f64e272eaf1561b5df9ee135a7187a
Merge "delete dsh group "mediawiki-installation-precise"" into production