Nobody needs to use 2.6 any more, so reduce the maintenance burden.

https://lists.freedesktop.org/archives/libreoffice/2018-March/079670.html

Change-Id: I101e26fbceffbe6119f4a6484530f27760b03eb4
Reviewed-on: https://gerrit.libreoffice.org/50816Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
Tested-by: Jenkins <ci@libreoffice.org>