Fixes the MSVC build.

Fixes the MSVC build.
Commit r152704 exposed a latent MSVC limitation (aka bug).
Both ilist and and iplist contains the same function:
  template<class InIt> void insert(iterator where, InIt first, InIt last) {
    for (; first != last; ++first) insert(where, *first);
  }

Also ilist inherits from iplist and ilist contains a "using iplist<NodeTy>::insert".
MSVC doesn't know which one to pick and complain with an error.

I think it is safe to delete ilist::insert since it is redundant anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152746 91177308-0d34-0410-b5e6-96231b3b80d8