[xref cpp23] Remove reference to diff against C++23

Oops.  Clearly the section describing the diffs against C++23
was not present in C++23 itself.

Author: AlisdairM

source/xrefdelta.tex

@@ -96,7 +96,6 @@
 \movedxref{diff.cpp11.dcl.dcl}{diff.cpp11.dcl}
 \movedxref{diff.cpp14.dcl.dcl}{diff.cpp14.dcl}
 \movedxref{diff.cpp17.dcl.dcl}{diff.cpp17.dcl}
-\movedxref{diff.cpp23.dcl.dcl}{diff.cpp23.dcl}
 \movedxref{diff.stat}{diff.stmt}
 \movedxref{stmt.stmt}{stmt}