btrfs: remove unused BTRFS_IOPRIO_READA

The last user of this definition was removed in patch f26c9238
("btrfs: remove reada infrastructure") so we can remove this definition.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -87,9 +87,6 @@ struct reloc_control;
 
 #define BTRFS_EMPTY_DIR_SIZE 0
 
-/* ioprio of readahead is set to idle */
-#define BTRFS_IOPRIO_READA (IOPRIO_PRIO_VALUE(IOPRIO_CLASS_IDLE, 0))
-
 #define BTRFS_DIRTY_METADATA_THRESH	SZ_32M
 
 #define BTRFS_MAX_EXTENT_SIZE SZ_128M