Stop keeping etc/refcards generated PDFs in the repository

Stop keeping etc/refcards generated PDFs in the repository

They are on the gnu.org website and will be in the release tarfiles.

* admin/make-tarball.txt: Mention generating pdfs in etc/refcards.

* etc/refcards/calccard.pdf, etc/refcards/cs-dired-ref.pdf:
* etc/refcards/cs-refcard.pdf, etc/refcards/de-refcard.pdf:
* etc/refcards/dired-ref.pdf, etc/refcards/fr-dired-ref.pdf:
* etc/refcards/fr-refcard.pdf, etc/refcards/gnus-booklet.pdf:
* etc/refcards/gnus-refcard.pdf, etc/refcards/orgcard.pdf:
* etc/refcards/pl-refcard.pdf, etc/refcards/pt-br-refcard.pdf:
* etc/refcards/refcard.pdf, etc/refcards/ru-refcard.pdf:
* etc/refcards/sk-dired-ref.pdf, etc/refcards/sk-refcard.pdf:
Remove generated files from repository.