distro/test: turn off OBS packaging tests for CentOS7

Builds are still checked by the other pkftest suite. However, OBS
mirrors for CentOS 7 are just problematic. We've already tried to
contact them once, they fixed the issue but mentioned it will probably
come back. No point in wasting any more time with this test then.

.gitlab-ci.yml

@@ -605,13 +605,6 @@ obs:build:all:
     when:
       - script_failure
 
-obs:centos7:x86_64:
-  <<: *distrotest
-  variables:
-    OBS_REPO: knot-resolver-latest
-    DISTROTEST_NAME: centos7
-    DISTROTEST_REPO: CentOS_7_EPEL
-
 obs:centos8:x86_64:
   <<: *distrotest
   allow_failure: true