[lwip] delete useless RTM export (#5743)
[lwip] delete useless RTM export (#5743)


RT-Thread/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread