[depr.default.allocator] Index allocator::is_always_equal here (#5152)

LWG3170 deprecated `allocator::is_always_equal`. We moved it to Annex D, but left the index entry behind.

Author: CaseyCarter

source/future.tex

@@ -1319,6 +1319,7 @@
 The following member is defined in addition to those
 specified in \ref{default.allocator}:
 
+\indexlibrarymember{is_always_equal}{allocator}%
 \begin{codeblock}
 namespace std {
   template<class T> class allocator {

source/utilities.tex

@@ -8700,7 +8700,6 @@
 \indexlibrarymember{size_type}{allocator}%
 \indexlibrarymember{difference_type}{allocator}%
 \indexlibrarymember{propagate_on_container_move_assignment}{allocator}%
-\indexlibrarymember{is_always_equal}{allocator}%
 \indexlibrarymember{operator=}{allocator}%
 \begin{codeblock}
 namespace std {