You could probably just do
sudo apt install --reinstall mysql-common
If that doesn't work, download mysql-common.deb from here, choose the right version, open it with Archive Manager, and extract the directory from it. You may need to create a couple of symbolic links. On my system they are:
/etc/mysql/my.cnf -> /etc/alternatives/my.cnf
/etc/alternatives/my.cnf -> /etc/mysql/mysql.cnf (this link probably still exists)
although the latest versions of mysql come with a version of mysql.cnf in /etc/mysql/conf.d. So check what /etc/alternatives/my.cnf points to.
