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 #12961

manage_plugin_uninstall.php

@@ -63,8 +63,6 @@
 
 if( !is_null( $t_plugin ) ) {
 	plugin_uninstall( $t_plugin );
-} else {
-	plugin_force_uninstall( $f_basename );
 }
 
 form_security_purge( 'manage_plugin_uninstall' );