Update the ProgrammersManual explanation for ilist and iplist

[lanza]

Summary

They are now using aliases and thus the comments about iplist are now
incorrect. Remove them here.

[dexonsmith]

LGTM.

[lanza]

rebase

llvm/docs/ProgrammersManual.rst

 
 ``ilist`` has the same drawbacks as ``std::list``, and additionally requires an
 ``ilist_traits`` implementation for the element type, but it provides some novel
 characteristics.  In particular, it can efficiently store polymorphic objects,
 the traits class is informed when an element is inserted or removed from the
 list, and ``ilist``\ s are guaranteed to support a constant-time splice
 operation.
 
+An ``ilist`` and an ``iplist`` are ``using`` aliases to one another and the
+latter only currently exists for historical purposes.
+
 These properties are exactly what we want for things like ``Instruction``\ s and
 basic blocks, which is why these are implemented with ``ilist``\ s.
 
 Related classes of interest are explained in the following subsections:
 
 * :ref:`ilist_traits <dss_ilist_traits>`
 
-* :ref:`iplist <dss_iplist>`
-
 * :ref:`llvm/ADT/ilist_node.h <dss_ilist_node>`
 
 * :ref:`Sentinels <dss_ilist_sentinel>`
 
 .. _dss_packedvector:
 
 llvm/ADT/PackedVector.h
 ^^^^^^^^^^^^^^^^^^^^^^^
State get() {
       return Vec1[0]; // returns 'Both'.
   }
 
 .. _dss_ilist_traits:
 
 ilist_traits
 ^^^^^^^^^^^^
 
-``ilist_traits<T>`` is ``ilist<T>``'s customization mechanism. ``iplist<T>``
-(and consequently ``ilist<T>``) publicly derive from this traits class.
-
-.. _dss_iplist:
-
-iplist
-^^^^^^
-
-``iplist<T>`` is ``ilist<T>``'s base and as such supports a slightly narrower
-interface.  Notably, inserters from ``T&`` are absent.
-
-``ilist_traits<T>`` is a public base of this class and can be used for a wide
-variety of customizations.
+``ilist_traits<T>`` is ``ilist<T>``'s customization mechanism. ``ilist<T>``
+publicly derives from this traits class.
 
 .. _dss_ilist_node:
 
 llvm/ADT/ilist_node.h
 ^^^^^^^^^^^^^^^^^^^^^
 
 ``ilist_node<T>`` implements the forward and backward links that are expected
 by the ``ilist<T>`` (and analogous containers) in the default manner.