.gitlab-ci.yml: Only install modern MySQL packages

Our Perl images are now all based on Debian Buster so there is no longer
a need to try and install Jessie packages.

.gitlab-ci.yml

@@ -18,7 +18,7 @@
   before_script:
     - apt-get update
     - apt-get install -y build-essential cpanminus git
-    - apt-get install -y libmysqlclient-dev mysql-client || apt-get install -y default-libmysqlclient-dev default-mysql-client
+    - apt-get install -y default-libmysqlclient-dev default-mysql-client
     - apt-get install -y libssl-dev sqlite3
     - git clone --branch=master --depth=1 https://github.com/Ensembl/ensembl.git
     - git clone --branch=release-1-6-924 --depth=1 https://github.com/bioperl/bioperl-live.git