Why did you unpack the plugin from it's phar? , The default for a phar plugin is to load it as a phar, You'll have to try disabling the plugin in mysql check the ost_plugin table for the plugin and change isactive to 0.
If you still can't load the admin interface, a plugin shouldn't do that!, so I'm guessing you've overridden files in the default install, you might want to check that, download a fresh copy and make a backup of your config then repair osticket.