mass email sender install gives error

I tried installing the mass email sender. After I uploaded the zip file. I recieved the following error. I'am unable to log into the back end to revmove the code.

Fatal error: Call to undefined function is_site_admin() in /home/forhism3/public_html/forum/wp-content/plugins/128115207061/update-notifications.php on line 417

How should I proceed to uninstall the mass email sender plugin? Thank you for your help