libstdc++: Remove obsolete comment in <string> header

     The comment is obsolete because char_traits.h do not include 
stl_algobase.h
     anymore and stl_algobase.h is included directly from <string> a few 
lines
     below.

     libstdc++-v3/ChangeLog:

             * include/std/string: Remove obsolete comment about 
char_traits.h including
             stl_algobase.h.

François