Updated the diag() spec.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402719

dlib/matrix/matrix_utilities_abstract.h

@@ -22,8 +22,6 @@ namespace dlib
         const matrix_exp& m
     );
     /*!
-        requires
-            - m is a square matrix
         ensures
             - returns a column vector R that contains the elements from the diagonal 
               of m in the order R(0)==m(0,0), R(1)==m(1,1), R(2)==m(2,2) and so on.