Can't enable mpm_prefork with Apache 2.4 on Ubuntu 14.04
You can find it by grepping through the whole Apache configuration:
grep -r mpm_event_module /etc/apache2
It's almost certainly in /etc/apache2/mods-available/mpm_event.load. In Debian/Ubuntu the standard way of enabling and disabling Apache modules is to run e.g.
a2dismod mpm_event
a2enmod mpm_prefork
See man a2enmod, and similarly man a2enconf and man a2ensite.