btrfs: remove btree_readpage

There is no way for this function to be called as ->readpage() since
it's called from
generic_file_buffered_read/filemap_fault/do_read_cache_page/readhead
code. BTRFS doesn't utilize the first 3 for the btree inode and
implements it's owon readhead mechanism. So simply remove the function.
Reviewed-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -948,11 +948,6 @@ static int btree_writepages(struct address_space *mapping,
 	return btree_write_cache_pages(mapping, wbc);
 }
 
-static int btree_readpage(struct file *file, struct page *page)
-{
-	return extent_read_full_page(page, btree_get_extent, 0);
-}
-
 static int btree_releasepage(struct page *page, gfp_t gfp_flags)
 {
 	if (PageWriteback(page) || PageDirty(page))
@@ -992,7 +987,6 @@ static int btree_set_page_dirty(struct page *page)
 }
 
 static const struct address_space_operations btree_aops = {
-	.readpage	= btree_readpage,
 	.writepages	= btree_writepages,
 	.releasepage	= btree_releasepage,
 	.invalidatepage = btree_invalidatepage,