Remove `add_settings_error()` logic in `WPSEO_Option_Social::validate_option()`

[leonidasmi]

As per an investigation when renaming Twitter to X, we probably no longer use the part where we

if ( function_exists( 'add_settings_error' ) ) {
	add_settings_error(
		$this->group_name, // Slug title of the setting.
		$key, // Suffix-ID for the error message box.
		sprintf(
			/* translators: %s expands to a twitter user name. */
			__( '%s does not seem to be a valid Twitter Username. Please correct.', 'wordpress-seo' ),
			'<strong>' . esc_html( sanitize_text_field( $dirty[ $key ] ) ) . '</strong>'
		), // The error message.
		'error' // Message type.
	);
}

as we no longer save Social Media links using the settings API.

Let's confirm the above and if so, remove this logic.

Also, maybe there's also other places where add_settings_error() is now obsolete?