Fix 'Undefined variable' PHP notice in API REST

Since the Slim Container's 'displayErrorDetails' setting is set to true
according to $t_show_detailed_errors's value, instead of referencing the
variable directly within the closure via a 'use' statement, we retrieve
the Container's settings to determine whether PHP exception details
should be shown or not.

Remove $t_show_detailed_errors variable as it is no longer needed.

Regression introduced by b2119ce.

Fixes #25429

api/rest/index.php

@@ -46,8 +46,7 @@
 # Show SLIM detailed errors according to Mantis settings
 $t_config = array();
 
-$t_show_detailed_errors = ON == config_get_global( 'show_detailed_errors' );
-if( $t_show_detailed_errors ) {
+if( ON == config_get_global( 'show_detailed_errors' ) ) {
 	$t_config['settings'] = array( 'displayErrorDetails' => true );
 }
 
@@ -76,7 +75,8 @@
 		$t_error_to_log =  $p_exception->getMessage() . "\n" . $t_stack_as_string;
 		error_log( $t_error_to_log );
 
-		if( $t_show_detailed_errors ) {
+		$t_settings = $p_container->get('settings');
+		if( $t_settings['displayErrorDetails'] ) {
 			$p_response = $p_response->withJson( $t_data );
 		}