Fix pipelining issue with multipart upload

Do not clear buffer at the end of multipart request to allow following request to proceed.

Closes https://github.com/cesanta/mongoose/pull/940

CL: mg: Fix pipelining issue with multipart upload

PUBLISHED_FROM=cc7d38b126eaa863553ee4e124cfafcd72030fcf

src/mg_http.c

@@ -1159,7 +1159,6 @@ static void mg_http_multipart_continue(struct mg_connection *c) {
         break;
       }
       case MPS_FINISHED: {
-        mbuf_remove(&c->recv_mbuf, c->recv_mbuf.len);
         return;
       }
     }