lxc, schroot: drop explicit --apt-upgrade

That's now always passed by debci-autopkgtest
4 jobs for master in 10 minutes and 4 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #220175
ruby-style

00:01:57

passed #220174
stable

00:09:45

passed #220173
testing

00:10:04

passed #220172
unstable

00:09:27