runtime: remove unnecessary dcache flush.

Data cache is write-through, so sending data to DMA doesn't need
a flush.

artiq/firmware/runtime/session.rs

@@ -410,7 +410,6 @@ fn process_kern_message(io: &Io, mut stream: Option<&mut TcpStream>,
             }
             &kern::DmaRecordStop(name) => {
                 session.congress.dma_manager.record_stop(name);
-                board::cache::flush_cpu_dcache();
                 board::cache::flush_l2_cache();
                 kern_acknowledge()
             }