Remove OA_DANGEROUS from sort

Remove OA_DANGEROUS from sort

OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:

($a, $b) = ($b, $a);

In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.

pp_sort returns its arguments.  aassign_common_vars will check its kid
ops for danger as well, so it’s not necessary for sort itself to be
flagged this way.  This will allow cases like ($a,$b) = sort($c,$d) to
forego the temp copy.