block: kill blk_flush_plug_list() export

With all drivers and file systems converted, we only have
in-core use of this function. So remove the export.
Reporteed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-core.c

@@ -2787,7 +2787,6 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 
 	local_irq_restore(flags);
 }
-EXPORT_SYMBOL(blk_flush_plug_list);
 
 void blk_finish_plug(struct blk_plug *plug)
 {