block: fix kerneldoc comment for blk_attempt_plug_merge()

Commit 5f0ed774 ("block: sum requests in the plug structure") removed
the request_count parameter from block_attempt_plug_merge(), but did not
remove the associated kerneldoc comment, introducing this warning to the
docs build:

  ./block/blk-core.c:685: warning: Excess function parameter 'request_count' description in 'blk_attempt_plug_merge'

Remove the obsolete description and make things a little quieter.
Signed-off-by: NJonathan Corbet <corbet@lwn.net>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -661,7 +661,6 @@ bool bio_attempt_discard_merge(struct request_queue *q, struct request *req,
  * blk_attempt_plug_merge - try to merge with %current's plugged list
  * @q: request_queue new bio is being queued at
  * @bio: new bio being queued
- * @request_count: out parameter for number of traversed plugged requests
  * @same_queue_rq: pointer to &struct request that gets filled in when
  * another request associated with @q is found on the plug list
  * (optional, may be %NULL)