[otrs] How to disable customers from changing ticket priority

[OTRS 88]

In older versions of OTRS there was a parameter called CustomerPriority 
that you could set to 1 or 0 to allow or disallow customers to change 
the priority of their tickets. Now that CustomerPriority is not longer 
around, how is that feature accessed? In other words, how do I make a 
change to allow/disallow customers from changing their ticket priorities?