Always unlink pending write requests when entering close_wait
Always unlink pending write requests when entering close_wait

Seen on a stream in `H2O_HTTP3_SERVER_STREAM_STATE_CLOSE_WAIT`:
```
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /build/lib/core/request.c:406:62 in
h2o: /build/lib/http3/server.c:988: void run_delayed(h2o_timer_t *): Assertion `stream->req_body != NULL' failed.
received fatal signal 6
```

h2o/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,cwyang/h2o,h2o/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,cwyang/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,h2o/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,h2o/h2o,yannick/h2o,yannick/h2o,h2o/h2o