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  Remove the out of date file.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: NFrank Rowand <frank.rowand@sony.com>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/todo.txt

-Todo list for devicetree:
-
-=== General structure ===
-- Switch from custom lists to (h)list_head for nodes and properties structure
-
-=== CONFIG_OF_DYNAMIC ===
-- Switch to RCU for tree updates and get rid of global spinlock
-- Document node lifecycle for CONFIG_OF_DYNAMIC
-- Always set ->full_name at of_attach_node() time
-- pseries: Get rid of open-coded tree modification from arch/powerpc/platforms/pseries/dlpar.c