block: drop write notifiers

They are unused now.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210506090621.11848-3-vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -400,7 +400,6 @@ BlockDriverState *bdrv_new(void)
     for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
         QLIST_INIT(&bs->op_blockers[i]);
     }
-    notifier_with_return_list_init(&bs->before_write_notifiers);
     qemu_co_mutex_init(&bs->reqs_lock);
     qemu_mutex_init(&bs->dirty_bitmap_mutex);
     bs->refcnt = 1;

block/io.c

@@ -3165,12 +3165,6 @@ bool bdrv_qiov_is_aligned(BlockDriverState *bs, QEMUIOVector *qiov)
     return true;
 }
 
-void bdrv_add_before_write_notifier(BlockDriverState *bs,
-                                    NotifierWithReturn *notifier)
-{
-    notifier_with_return_list_add(&bs->before_write_notifiers, notifier);
-}
-
 void bdrv_io_plug(BlockDriverState *bs)
 {
     BdrvChild *child;

include/block/block_int.h

@@ -954,9 +954,6 @@ struct BlockDriverState {
      */
     int64_t total_sectors;
 
-    /* Callback before write request is processed */
-    NotifierWithReturnList before_write_notifiers;
-
     /* threshold limit for writes, in bytes. "High water mark". */
     uint64_t write_threshold_offset;
 
@@ -1083,15 +1080,6 @@ void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
 bool bdrv_backing_overridden(BlockDriverState *bs);
 
 
-/**
- * bdrv_add_before_write_notifier:
- *
- * Register a callback that is invoked before write requests are processed but
- * after any throttling or waiting for overlapping requests.
- */
-void bdrv_add_before_write_notifier(BlockDriverState *bs,
-                                    NotifierWithReturn *notifier);
-
 /**
  * bdrv_add_aio_context_notifier:
  *