gerrit: remove mysql-client-core-5.7 package

This doesn't install of Focal; moving forward we'll either use H2 or a
container database.  Just remove this package for now.

Change-Id: I69cdcdddc1ba0e0cf4ef5f8ba705bcd3a2afa689

playbooks/roles/gerrit/tasks/main.yaml

@@ -255,12 +255,11 @@
     name: '[{{ gerrit_vhost_name }}]:29418'
   when: gerrit_self_hostkey is defined
 
-- name: Install apache2 and mysql client tools
+- name: Install apache2
   apt:
     name:
       - apache2
       - apache2-utils
-      - mysql-client-core-5.7
     state: present
 
 - name: Apache modules