The file you want is ->ticket.open.inc.php. At line 278, after the <tr>, type this in:
<? php /* (Not sure if the code will come out on the post)
and at line 296 before the <tr>, type this in:
This basically comments out the code that displays the SLA plan. Since the SLA plan field does not require a response from the user (I am not 100% sure on that, I just know I do not have to enter anything in my installation) it will still work.
I literally just did this and tested. Works for me.