How to remove buttons from the WP Editor on the edit post page in the admin area

you can custom the editor with the filter tiny_mce_before_init

e.g. try this code

add_filter("tiny_mce_before_init", function ($initArray, $editor_id) {

    $initArray["toolbar1"] = "bold,italic,underline,link,unlink,spellchecker";
    $initArray["toolbar2"] = "";


    return $initArray;

}, 10, 2);