[docs] remove some out-of-date content in LLVM Programmer's Manual (#74989)

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

This part of the manual was written 14 years ago, in:
https://github.com/llvm/llvm-project/commit/37027c30ec526afe3bb571df6f8701bf0d322f22

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:
https://github.com/llvm/llvm-project/commit/f197b1f78f854d8513ef617b8cfc61860f7b4b84

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