rhi: Remove broken assert

The recent patch for enabling the command buffer be specified already
during the sync phase changes the behavior so that the
m_currentFrameCommandBuffer may already contain a non-null value when we
get to the render phase. Just remove the assert.

Change-Id: I7f1e0d8cd6a307f64425ac4ce9dd056a89e7eed2
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/quick/scenegraph/qsgdefaultrendercontext.cpp

@@ -242,8 +242,6 @@ void QSGDefaultRenderContext::beginNextRhiFrame(QSGRenderer *renderer, QRhiRende
                                                 RenderPassCallback mainPassRecordingEnd,
                                                 void *callbackUserData)
 {
-    Q_ASSERT(!m_currentFrameCommandBuffer);
-
     renderer->setRenderTarget(rt);
     renderer->setRenderPassDescriptor(rp);
     renderer->setCommandBuffer(cb);