ci/b2c: tags are mandatory

Jobs without tags are not accepted, so there is no point in having
support for creating them.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26653>

.gitlab-ci/b2c/b2c.yml.jinja2.jinja2

@@ -2,12 +2,10 @@ version: 1
 
 # Rules to match for a machine to qualify
 target:
-{% if tags %}
   tags:
 {% for tag in tags %}
     - '{{ tag | trim }}'
 {% endfor %}
-{% endif %}
 
 timeouts:
   first_console_activity:  # This limits the time it can take to receive the first console log