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The comment is obsolete because char_traits.h do not include stl_algobase.h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.

libstdc++-v3/include/std/string

@@ -37,7 +37,7 @@
 
 #include <bits/c++config.h>
 #include <bits/stringfwd.h>
-#include <bits/char_traits.h>  // NB: In turn includes stl_algobase.h
+#include <bits/char_traits.h>
 #include <bits/allocator.h>
 #include <bits/cpp_type_traits.h>
 #include <bits/localefwd.h>    // For operators >>, <<, and getline.