Revert "Define WIN32_LEAN_AND_MEAN to avoid collision with winsock2"

The Windows builds have been missing some libraries, notably
avdevice-58.dll, avfilter-7.dll, and prostproc-55.dll as reported by a
user on Discord.  I suspect it's caused by cc842fd, although I don't
fully understand why that would happen.

Author: pstef

ui/drivers/ui_qt.cpp

@@ -14,7 +14,6 @@
  * If not, see <http://www.gnu.org/licenses/>.
  */
 
-#define WIN32_LEAN_AND_MEAN
 #include <QApplication>
 #include <QAbstractEventDispatcher>
 #include <QtWidgets>