btrfs: remove stale definition of BUFFER_LRU_MAX

Long time ago (2008), the extent buffers were organized in a LRU list
and switched to rb-tree in 6af118ce51 ("Btrfs: Index extent
buffers in an rbtree"). There was one stale macro definition left.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -109,8 +109,6 @@ static inline void __btrfs_debug_check_extent_io_range(const char *caller,
 #define btrfs_debug_check_extent_io_range(c, s, e)	do {} while (0)
 #endif
 
-#define BUFFER_LRU_MAX 64
-
 struct tree_entry {
 	u64 start;
 	u64 end;