Remove cl_null_string defintion.

It's been unused since the last release.

src/Makefile.am

@@ -100,7 +100,6 @@ base/string/cl_st_make0.cc \
 base/string/cl_st_make0.h \
 base/string/cl_st_make1.cc \
 base/string/cl_st_make2.cc \
-base/string/cl_st_null.cc \
 base/string/input/cl_st_get1.cc \
 base/string/input/cl_st_get2.cc \
 base/string/input/cl_st_getline1.cc \

src/base/string/cl_st_null.cc

@@ -1,23 +0,0 @@
-// cl_null_string.
-
-// General includes.
-#include "base/cl_sysdep.h"
-
-// Specification.
-#include "cln/string.h"
-
-namespace cln {
-
-#warning "Please remove me"
-extern const cl_string cl_null_string;
-
-}  // namespace cln
-
-// Implementation.
-
-namespace cln {
-
-const cl_string cl_null_string = cl_string(NULL,0);
-
-}  // namespace cln
-