build: Remove unused AC_CHECK_DECLS for strnlen

This is not used since we moved away from the original strnlen.h header
in commit d73c292dab981991d73bb3678b0ee13f664c869a.

configure.ac

@@ -108,7 +108,6 @@ DPKG_CHECK_PROGNAME
 DPKG_CHECK_COMPAT_FUNCS([getopt getopt_long obstack_free \
                          strnlen strndup strerror strsignal asprintf \
                          scandir alphasort unsetenv])
-AC_CHECK_DECLS([strnlen], [[#include <string.h>]])
 AC_CHECK_FUNCS([memcpy lchown],
                [], [AC_MSG_ERROR([missing required function])])
 AC_CHECK_FUNCS([strtoimax isascii setsid getdtablesize \