Remove one-time variable from add_settings_error

[niallkennedy]

Append an array to $wp_settings_errors without assigning it to a variable in the previous line.

Before:
$var = array( 'type' => $type ... );
$wp_settings_errors[] = $var;

After:
$wp_settings_errors[] = array( 'type' => $type ... );

[niallkennedy]

append array directly to $wp_settings_errors

[obenland]

Refreshed patch

[SergeyBiryukov]

In 28681:

Remove one-time variable from add_settings_error().

props niallkennedy.
fixes #22228.