compat: remove strnlen back-compat code

This was needed for mingw (not mingw-w64), and some older versions of
macOS, which we no-longer support.

configure.ac

@@ -984,7 +984,6 @@ AC_CHECK_DECLS([getifaddrs, freeifaddrs],[CHECK_SOCKET],,
     [#include <sys/types.h>
     #include <ifaddrs.h>]
 )
-AC_CHECK_DECLS([strnlen])
 
 dnl These are used for daemonization in bitcoind
 AC_CHECK_DECLS([fork])