remove comma before ‘and’

That’s actually an Oxford Comma and is semantically correct.

If you don’t want that, you’d have to handle the display output yourself. As I’m not sure how you’re outputting that list, it’s hard to point you in the right direction, but if you’re using a Pods Template, you can use an each loop:

[if relationship_field]
<ul class="list-field">
[each relationship_field]
<li>{@post_title}</li>
[/each]
</ul>
[/if]

Then you can apply CSS to that list to control comma’s and etc.
Here’s one with the Oxford comma: https://codepen.io/jimtrue/pen/RYXQNL
and one without: https://codepen.io/jimtrue/pen/ZjJVGp