Change “Display Site Title and Tagline” checkbox text in theme customizer

You will need to de-register that control. It will look like this:

/**
 * Remove parts of the Options menu we don't use.
 *
 * @param WP_Customize_Manager $wp_customize Customizer manager.
 */
function de_register( $wp_customize ) {
    $wp_customize->remove_control('display_header_text');
}
add_action( 'customize_register', 'de_register', 11 );

For more information, this Theme Customization is a good start.