Drop "unstable" Debian "bullseye" unit and system jobs

Drop "unstable" Debian "bullseye" unit and system jobs

"Unstable" Debian "bullseye" unit and system jobs differ from unit and
system jobs on the Debian "bullseye" image by running a few more tests
enabled by the CI_ENABLE_ALL_TESTS environmental variable.  These tests
are no less stable than the rest, and thus to have distinct jobs for
them wastes resources.

Run "unstable" unit and system tests as part of stock Debian "bullseye"
jobs.

(cherry picked from commit 2b9111b4358c0183eee03082e025fea37a11e852)