QFormDataBuilder: make default constructor implicit

Default constructors should be implicit. No Q_IMPLICIT needed.

Found in API review.

Change-Id: Id0ca95102330a345b58bdf5641cf07bae0a8c6b3
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
Reviewed-by: Mate Barany <mate.barany@qt.io>
(cherry picked from commit ed1774584d0231fadda0e9197d92ddcdb6bf2ea3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/access/qformdatabuilder.h

@@ -93,7 +93,7 @@ private:
 class QFormDataBuilder
 {
 public:
-    Q_NETWORK_EXPORT explicit QFormDataBuilder();
+    Q_NETWORK_EXPORT QFormDataBuilder();
 
     QFormDataBuilder(QFormDataBuilder &&other) noexcept : d_ptr(std::exchange(other.d_ptr, nullptr)) {}