Remaining .dhelp files prevent directory removing on purge

[Peter S Galbraith <GalbraithP@dfo-mpo.gc.ca>]

I have a package (gri-html-doc) which used to provide documentation in
/usr/share/doc/gri-html-doc/html/ and I have sinced moved the docs to
/usr/share/doc/gri/html/.

Upon upgrading from one version to the other, I get the warning:

dpkg: warning - unable to delete old file `/usr/share/doc/gri-html-doc/html':
 Directory not empty

That directory contains the old .dhelp file, which I didn't create; I
use a doc-base file.

Was I suppose to delete /usr/share/doc/gri-html-doc/html/.dhelp
in my old package's prerm?

Since it's too late to do that, do I delete it and the html directory
(if empty) in the new package's preinst?  Or even in the postinst?

Thanks,
Peter