Scaffold theme test should work in PHP greater than 8.0

[ernilambar]

This PR removes following code from theme-bootstrap.mustache:

if ( PHP_MAJOR_VERSION >= 8 ) {
	echo "The scaffolded tests cannot currently be run on PHP 8.0+. See https://github.com/wp-cli/scaffold-command/issues/285" . PHP_EOL; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
	exit( 1 );
}

While working on #331 I noticed this conditional is now not needed.

CC @schlessera - Ref #285