MariaDB packages provide corresponding symbols with name mysql. It means that as soon as any package requires mysql, yum will actually choose mariadb package to be installed.
Remove all mariadb packages (it will remove also all dependencies, so consider doing this step carefully):
# yum remove 'mariadb*'
This task consists from several sub-tasks. After every sub-task you'll need to remove installed packages (like described in Prerequisites section above).
Try install mysql-server package:
# yum install mysql-server
Expected result: packages mariadb-server mariadb-libs and mariadb will be installed.
Try install perl-DBD-MySQL package:
# yum install perl-DBD-MySQL
Expected result: packages perl-DBD-MySQL and mariadb-libs will be installed.
Try install akonadi-mysql package:
# yum install akonadi-mysql
Expected result: packages akonadi-mysql, mariadb-server, mariadb and mariadb-libs will be installed.