apt-get update produces the error

[Grizzly]

Tuesday, December 13, 2016  at 18:53, Colin Watson wrote:
Re: apt-get update produces the err (at least in part)

>On Tue, Dec 13, 2016 at 08:26:35AM -0000, Grizzly wrote:
>> When I run "apt-get update" it sems to work but ends with
>> 
>> W: The repository 'http://extras.ubuntu.com/ubuntu xenial Release' does not have a Release file.
>> N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
>> N: See apt-secure(8) manpage for repository creation and user configuration details.
>> E: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/xenial/main/source/Sources  404  Not Found [IP: 91.189.92.152 80]
>> E: Some index files failed to download. They have been ignored, or old ones used instead.

>extras.ubuntu.com was retired in the Ubuntu 15.04 cycle:

>  https://lists.ubuntu.com/archives/technical-board/2015-January/002063.html

>Just remove it from sources.list.

As far as I could see, the only Reps I had check marks against (in 
Software.gtk) were Xenial Main Universe Restricted Multiverse

or that's what 
cat /etc/apt/sources.list
shows