of: Documentation: remove unmaintained todo file

The todo.txt file was created by a previous maintainer and has
never been updated by the current OPEN FIRMWARE AND FLATTENED
DEVICE TREE maintainers.  Remove the out of date file.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>