webadmin: 'remove' confirmation dialog message cleanup

Before this patch, 'remove' confirmation dialog message contained a default message
that could have been overridden only if the dialog's HashName didn't start with "remove_".

In this patch:

 * 'remove' confirmation dialog still contains a default message, but now it can always be
   overridden, regardless of the dialog's hashName value.

 * Cleaned up (now unnecessary) HashName manipulation in RemoveNetworkQoSModel.

 * Cleaned up useless calls to set dialog's message (which were probably useful up until
   http://gerrit.ovirt.org/#/c/14621/ has been merged) in order to prevent regressions such
   as displaying "Virtual Machines" as the dialog's content instead of the default
   "Are you sure you want to remove the following items?" message.

 * Removed the related (now unused) UI constants.

Change-Id: I7411dec0eaf8d60cefff563988618f7c548eecce
Bug-Url: https://bugzilla.redhat.com/1021584
Signed-off-by: Greg Sheremeta <gshereme@redhat.com>