rgw: remove meaningless oid_rand

We have no need to genarate an `oid_rand` and pass it into the `prepare`
function, which will do nothing with `oid_rand` as following:

```
  virtual int prepare(RGWRados *_store, string *oid_rand) {
    store = _store;
    return 0;
  }
```
On the other hand, the `RGWObjManifest::generator::create_begin` will
genarate a different rand `oid` suffix as well as we need.

Signed-off-by: Yan Jun <yan.jun8@zte.com.cn>

src/rgw/rgw_op.cc

@@ -2709,12 +2709,7 @@ void RGWPutObj::execute()
       dispose_processor(processor);
       processor = select_processor(*static_cast<RGWObjectCtx *>(s->obj_ctx), &multipart);
 
-      string oid_rand;
-      char buf[33];
-      gen_rand_alphanumeric(store->ctx(), buf, sizeof(buf) - 1);
-      oid_rand.append(buf);
-
-      op_ret = processor->prepare(store, &oid_rand);
+      op_ret = processor->prepare(store, NULL);
       if (op_ret < 0) {
         ldout(s->cct, 0) << "ERROR: processor->prepare() returned "
 			 << op_ret << dendl;