EmscriptenMouseEvent has no 'timestamp' member anymore

Removed timestamp field from mouse, wheel, devicemotion and deviceorientation events. The presence of a timestamp on these events was slightly arbitrary, and populating this field caused a small profileable overhead that all users might not care about. It is easy to get a timestamp of an event by calling emscripten_get_now() or emscripten_performance_now() inside the event handler function of any event.

/qt5/qtbase/src/plugins/platforms/wasm/qwasmeventtranslator.cpp:549:34: error: no member named 'timestamp' in 'EmscriptenMouseEvent'
 auto timestamp = mouseEvent->timestamp;
 ~~~~~~~~~~ ^ 
/qt5/qtbase/src/plugins/platforms/wasm/qwasmeventtranslator.cpp:675:33: error: no member named 'timestamp' in 'EmscriptenMouseEvent'
 auto timestamp = mouseEvent.timestamp; 
 ~~~~~~~~~~ ^
2 errors generated.