Remove installation instructions specific to Ubuntu 15.10

Originally created by @sajolida on #11586 (Redmine)

According to https://en.wikipedia.org/wiki/List_of_Ubuntu_releases#Ubuntu_15.10_.28Wily_Werewolf.29, Ubuntu 15.10 is supported until 2016-07-28, so 2.8 (2016-12-13) looks like a good appromixation on when we should get rid of these on the website.

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