Remove markdown files for types that no longer exist

[bpow]

This is a component of the larger issue #192.

Types that were removed from the sheets still have markdown files in the docs repo. These need to be removed to accomplish #192, and if there is useful documentation material in those markdown files, then that needs to go elsewhere.

[bpow]

This is blocking from the standpoint of building/publishing documentation, since the processing code tries to look up types that no longer exist.