Remove Specific Buttons from WP Editor TinyMCE

The tinymce parameter allows you to pass configuration options directly to TinyMCE - see the docs for theme_advanced_buttons and theme_advanced_disable, and the button reference.

To show only the bold, italic and underline buttons:

wp_editor($value, "input...", array(    'tinymce' => array(        'theme_advanced_buttons1' => 'bold,italic,underline',        'theme_advanced_buttons2' => '',        'theme_advanced_buttons3' => ''    )));

Or, to show everything except the bold, italic and underline buttons:

wp_editor($value, "input...", array(    'tinymce' => array(        'theme_advanced_disable' => 'bold,italic,underline'    )));

As requested, your code modified:

add_action( 'gform_field_input', 'gforms_wp_editor', 10, 5 );function gforms_wp_editor( $input, $field, $value, $lead_id, $form_id ) {    if( $field["cssClass"] == 'richtext' ) {        ob_start();        wp_editor( $value, "input_{$form_id}_{$field['id']}",            array(                'media_buttons' => false,                'quicktags'     => false,                'textarea_name' => "input_{$field['id']}",                'tinymce'       => array(                    'theme_advanced_disable' => 'bold,italic,underline'                )            )        );        $input = ob_get_clean();    }    return $input;}