Do not run OpenBSD system test jobs for tags

OpenBSD virtual machines seem to affected particularly badly by other
activity happening on the host.  This causes trouble around release
time: when multiple tags are pushed to the repository, a large number of
jobs is started concurrently on all CI runners.  In extreme cases, this
causes the system test suite to run for about an hour (!) on OpenBSD
VMs, with multiple tests failing.  We investigated the test artifacts
for all such cases in the past and the outcome was always the same: test
failures were caused by extremely slow I/O on the guest.  We tried
various tricks to work around this problem, but nothing helped.

Given the above, stop running OpenBSD system test jobs for pending BIND
releases to prevent the results of these jobs from affecting the
assessment of a given release's readiness for publication.  This change
does not affect OpenBSD build jobs.  OpenBSD system test jobs will still
be run for scheduled and web-requested pipelines, to make sure we catch
any severe issues with test code on that platform sooner or later.

.gitlab-ci.yml

@@ -1141,7 +1141,6 @@ system:clang:openbsd6.6:amd64:
   needs: ["clang:openbsd6.6:amd64"]
   only:
     - schedules
-    - tags
     - web
 
 # Jobs with libtool disabled