btrfs: remove unused define WAIT_PAGE_LOCK for extent io

Last use was in the readahead code that got removed by f26c923
("btrfs: remove reada infrastructure").

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

fs/btrfs/extent_io.h

@@ -263,7 +263,6 @@ void free_extent_buffer(struct extent_buffer *eb);
 void free_extent_buffer_stale(struct extent_buffer *eb);
 #define WAIT_NONE	0
 #define WAIT_COMPLETE	1
-#define WAIT_PAGE_LOCK	2
 int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num,
 			     const struct btrfs_tree_parent_check *parent_check);
 static inline void wait_on_extent_buffer_writeback(struct extent_buffer *eb)