DOC: remove filenames usecase document

The API has already been implemented, there isn't anything substantial
left in this document that needs preserving.

Author: matthew-brett

doc/source/devel/usecases/filenames.rst

@@ -5,6 +5,5 @@
 .. toctree::
    :maxdepth: 2
 
-   filenames
    loading_saving
    spm_use