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 6af118ce

 ("Btrfs: Index extent
buffers in an rbtree"). There was one stale macro definition left.

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