Remove call to nonexistent plugin_force_uninstall()

This call has been present since manage_plugin_uninstall.php was
initially created, but the function has never been defined in Plugin
API.

Considering that having $t_plugin == null (supposedly meaning that the
plugin is not properly registered) is not a normal scenario, it makes
more sense to remove the call rather than creating the missing API.

Fixes 0012961