btrfs: remove unused map_private_extent_buffer

All uses of map_private_extent_buffer have been replaced by more
effective way. The set/get helpers have their own bounds checker.
The function name was confusing since the non-private helper was removed
in a6591715

 ("Btrfs: stop using highmem for extent_buffers") many
years ago.

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