Comment 110 for bug 1357093

stub,

I suppose the upgrading process did not change the settings concerning unattended-upgrade. You should do that manually after upgrading from 14.04 and initially remove extra kernels manually before upgrading.

de0u,

I tell about the restrictions concerning 'apt-get autoremove' in the wiki page. Likewise I tell about some alternatives not limited to 'purge-old-kernels'. I think it is harsh to say most things do not work, if you do not specify.
You could run Unattended-upgrade from command line to install new kernel updates when you want:

sudo apt-get update && sudo unattended-upgrade

As for un-clogging, did you read the 'Safely Removing Old Kernels' chapter? I see many people take time to complain, but I see none who is willing to spend even 1$ to get an easier solution for poor Ubuntu users.