editor.js

wp.domReady( () => {
	wp.blocks.unregisterBlockStyle( 'core/button', 'default' );
	wp.blocks.unregisterBlockStyle( 'core/button', 'outline' );
	wp.blocks.unregisterBlockStyle( 'core/button', 'squared' );
} );

functions.php

<?php

/**
 * Gutenberg scripts and styles
 * @see https://www.billerickson.net/wordpress-color-palette-button-styling-gutenberg
 */
function be_gutenberg_scripts() {
	wp_enqueue_script( 'be-editor', get_stylesheet_directory_uri() . '/assets/js/editor.js', array( 'wp-blocks', 'wp-dom' ), filemtime( get_stylesheet_directory() . '/assets/js/editor.js' ), true );
}
add_action( 'enqueue_block_editor_assets', 'be_gutenberg_scripts' );