slither-mutate: fix AOR mutator

[smonicas]

Before the AOR mutator could crash when an array was accessed through multi levels such as as a value of a mapping because the ir_expression.called.expression would be an Index operator. Removing the specific checks on the Identifier being an ArrayType make it works for all the possible cases with no practical downsides, maybe it could be a problem if there is a library with custom functions called pop and push used with the using-for directive

contract Counter {
    mapping(uint => uint[]) p;

    function s() public {
       p[1].push(2);
   }
}

[montyly]

is this removal expected?

[smonicas]

Yes. It was done here but the the CLI arg and README were not updated