Refactor stable branch jobs

Remove older stable branch jobs and variables that relied
on older Ubuntu images. Most of these branches are now EOL'd,
so we can remove the testing for them.

Change-Id: Iefc555162037b25e5b76d63b60f0ffb82896c3c9
Signed-off-by: Carlos da Silva <ces.eduardo98@gmail.com>
1 file changed