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.

src/compat/strnlen.cpp

@@ -1,18 +0,0 @@
-// Copyright (c) 2009-2018 The Bitcoin Core developers
-// Distributed under the MIT software license, see the accompanying
-// file COPYING or http://www.opensource.org/licenses/mit-license.php.
-
-#if defined(HAVE_CONFIG_H)
-#include <config/bitcoin-config.h>
-#endif
-
-#include <cstring>
-
-#if HAVE_DECL_STRNLEN == 0
-size_t strnlen( const char *start, size_t max_len)
-{
-    const char *end = (const char *)memchr(start, '\0', max_len);
-
-    return end ? (size_t)(end - start) : max_len;
-}
-#endif // HAVE_DECL_STRNLEN