gui: remove OpenSSL PRNG seeding (Windows, Qt only)

This removes the code introduced in [dashpay#4399](bitcoin#4399)
that attempts to add additional entroy to the OpenSSL PRNG using Windows messages.
Note that this is specific to bitcoin-qt running on Windows.

```
RAND_event() collects the entropy from Windows events such as mouse movements and other user interaction.
It should be called with the iMsg, wParam and lParam arguments of all messages sent to the window procedure.
It will estimate the entropy contained in the event message (if any), and add it to the PRNG.
The program can then process the messages as usual.
```

Besides BIP70, this is the last place we are directly using OpenSSL in the
GUI code. All other OpenSSL usage is in random.cpp.

Note that we are still also doing Windows specific entropy gathering in multiple
other places. Such as [RandAddSeedPerfmon](https://github.com/bitcoin/bitcoin/blob/master/src/random.cpp#L268)
and [RAND_screen()](https://github.com/bitcoin/bitcoin/blob/master/src/random.cpp#L600).

Also note that if RAND_event returns 0 (PRNG has NOT been seeded with enough data), we're
just logging a message and continuing on, which seems less than ideal.

src/qt/winshutdownmonitor.cpp

@@ -6,14 +6,11 @@
 
 #if defined(Q_OS_WIN)
 #include <shutdown.h>
-#include <util/system.h>
 
 #include <windows.h>
 
 #include <QDebug>
 
-#include <openssl/rand.h>
-
 // If we don't want a message to be processed by Qt, return true and set result to
 // the value that the window procedure should return. Otherwise return false.
 bool WinShutdownMonitor::nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult)
@@ -22,16 +19,6 @@ bool WinShutdownMonitor::nativeEventFilter(const QByteArray &eventType, void *pM
 
        MSG *pMsg = static_cast<MSG *>(pMessage);
 
-       // Seed OpenSSL PRNG with Windows event data (e.g.  mouse movements and other user interactions)
-       if (RAND_event(pMsg->message, pMsg->wParam, pMsg->lParam) == 0) {
-            // Warn only once as this is performance-critical
-            static bool warned = false;
-            if (!warned) {
-                LogPrintf("%s: OpenSSL RAND_event() failed to seed OpenSSL PRNG with enough data.\n", __func__);
-                warned = true;
-            }
-       }
-
        switch(pMsg->message)
        {
            case WM_QUERYENDSESSION: