[Pdl-porters] cleanup cruft in core-cleanup

[Chris Marshall]

PDL Developers-

There are a number of readme-type files in the PDL distribution
directories that no longer apply. For example, TODO lists that are a
decade out of date or applicable to a separate module that is now part
of the PDL distribution.

If you contributed or maintain a subdirectory or more of the PDL
distribution, please check out the core-cleanup branch and take this
opportunity to delete any vestigial files that are no longer relevant.
We'll be doing some more aggressive pruning going forwards but even
low-lying fruit is nice to get to.

Thanks much,
Chris