virtio-net: fix debug build due to missing try_lock_running()

The lock has been pushed down to a helper class and cannot be accessed here,
so remove the assert.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>

drivers/virtio-net.cc

@@ -619,8 +619,6 @@ int net::txq::try_xmit_one_locked(net_req* req)
     u16 vec_sz = 0;
     u64 tx_bytes = 0;
 
-    DEBUG_ASSERT(!try_lock_running(), "RUNNING lock not taken!\n");
-
     if (_parent->_mergeable_bufs) {
         req->mhdr.num_buffers = 0;
     }