QGrpcHttp2Channel: Remove unneeded check when creating stream

The `m_connection` check was not needed as there are two places where
this is called:
 - inside createHttp2Connection() -> connection gets created, 100% valid
 - inside processOperation(), where we need the m_connection check
   anyways.

Remove the check + error and replace it with an assert.

Pick-to: 6.10 6.9 6.8
Change-Id: Ib1d8a16db65cadfecb242f54a27ab70bb08a78fd
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/grpc/qgrpchttp2channel.cpp

@@ -1124,15 +1124,9 @@ void QGrpcHttp2ChannelPrivate::ensureSchemeIsValid(QLatin1String expected)
 bool QGrpcHttp2ChannelPrivate::createHttp2Stream(Http2Handler *handler)
 {
     Q_ASSERT(handler != nullptr);
+    Q_ASSERT(m_connection);
 
     auto *channelOpPtr = handler->operation();
-    if (!m_connection) {
-        operationContextAsyncError(channelOpPtr,
-                                   QGrpcStatus{ StatusCode::Unavailable,
-                                                tr("Unable to establish an HTTP/2 connection") });
-        return false;
-    }
-
     const auto streamAttempt = m_connection->createStream();
     if (!streamAttempt.ok()) {
         operationContextAsyncError(channelOpPtr,