[docs] remove some out-of-date content in LLVM Programmer's Manual. (#119565)

Remove the part about implicit conversion from an iterator to a pointer.

This part of the manual was written 14 years ago, in:
37027c3

There do exist a type casting operator in `ilist` then:
https://github.com/llvm/llvm-project/blob/37027c30ec526afe3bb571df6f8701bf0d322f22/llvm/include/llvm/ADT/ilist.h#L192-L194

But it has been remove since 2016:
f197b1f

So I think it makes sense to remove this part to avoid mislead new
contributors.

Author: yhgu2000