Revert ".gitlab-ci.yml: Use fedora-30 instead debian 9 for overnigth tests"

This reverts commit 08ead05bbe24a6de94e26b1121e5aa8dd341f691.

This commit was introduced to fix #1097, however it appears to not
be needed anymore after e48a03b2b226f1748edf5c2d5e28093da7ca6a1a which
uses a newer version of bst-external.

However, moving to fedora-30 runtime DID introduce a regression with
the ostree plugin, causing out-of-sync error messages to consistently
occur whenever fetching an ostree source.

Reverting to the old debian 9 docker image appears to fix the
ostree installation to work, fixing the afore mentioned regression
outlined in issue #1098.

This fixes #1098.
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index f09d204..fb5050b 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -96,7 +96,6 @@
 
 .overnight-tests: &overnight-tests-template
   stage: test
-  image: registry.gitlab.com/buildstream/buildstream-docker-images/testsuite-fedora:30-master-55318540
   variables:
     bst_ext_url: git+https://gitlab.com/BuildStream/bst-external.git
     bst_ext_ref: 0.16.0-0-gbe5683970abddbc4670592f79c473f4560b7be3a