Remove display of priority from _case_details as it seems unnecesary and creates a display issue

This is my attempt to address https://bugzilla.mozilla.org/show_bug.cgi?id=909475. I realize I'm probably mistaken, but I cannot see anywhere that we would need to display the priority via this template, as the priority should appear in the listing above the details. If simply removing this is a problem, please let me know where we do need it displayed.