btrfs: remove unused flag EXTENT_BUFFER_READ_ERR

This flag was added by commit 656f30db ("Btrfs: be aware of btree
inode write errors to avoid fs corruption") but it stopped being used
after commit 046b562b ("btrfs: use a separate end_io handler for
read_extent_buffer").

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>