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 2b9111b435)

.gitlab-ci.yml

@@ -714,7 +714,6 @@ unit:gcc:buster:amd64:
       artifacts: true
 
 # Jobs for regular GCC builds on Debian 11 "bullseye" (amd64)
-# (The second unit test job also executes unstable unit tests.)
 
 gcc:bullseye:amd64:
   variables:
@@ -730,31 +729,15 @@ gcc:bullseye:amd64:
 system:gcc:bullseye:amd64:
   <<: *debian_bullseye_amd64_image
   <<: *system_test_gcov_job
-  needs:
-    - job: unit:gcc:bullseye:amd64
-      artifacts: true
-
-system:gcc:bullseye:unstable:amd64:
-  <<: *debian_bullseye_amd64_image
-  <<: *system_test_job
-  <<: *api_schedules_triggers_web_triggering_rules
   variables:
     CI_ENABLE_ALL_TESTS: 1
   needs:
-    - job: gcc:bullseye:amd64
+    - job: unit:gcc:bullseye:amd64
       artifacts: true
 
 unit:gcc:bullseye:amd64:
   <<: *debian_bullseye_amd64_image
   <<: *unit_test_gcov_job
-  needs:
-    - job: gcc:bullseye:amd64
-      artifacts: true
-
-unit:gcc:bullseye:unstable:amd64:
-  <<: *debian_bullseye_amd64_image
-  <<: *unit_test_job
-  <<: *api_schedules_triggers_web_triggering_rules
   variables:
     CI_ENABLE_ALL_TESTS: 1
   needs: