Solution:
You can useĀ removeClass();
$('.bookingbutton2').removeClass('wp-dark-mode-ignore');
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<button type="button" class="bookingbutton2 wp-dark-mode-ignore" style="">
<span>Reserve Now</span>
</button>