Revert "wincng.c: fix clear memory feature compilation with mingw"

This reverts commit 2d2744e.

Autobuilds show that this did not solve the issue.
And it seems like RtlFillMemory is defined to memset,
which would be optimized out by some compilers.

src/wincng.c

@@ -67,15 +67,6 @@
 #include <wincrypt.h>
 #endif
 
-/* required for compilation of clear memory feature with mingw */
-#ifdef LIBSSH2_CLEAR_MEMORY
-#ifdef __MINGW32__
-#ifndef SecureZeroMemory
-#define SecureZeroMemory(ptr, cnt) RtlFillMemory(ptr, cnt, 0)
-#endif
-#endif
-#endif
-
 #define PEM_RSA_HEADER "-----BEGIN RSA PRIVATE KEY-----"
 #define PEM_RSA_FOOTER "-----END RSA PRIVATE KEY-----"
 #define PEM_DSA_HEADER "-----BEGIN DSA PRIVATE KEY-----"