Convert Czech and Slovakian refcards to UTF-8

* admin/notes/unicode: Don't mention Czech and Slovakian refcards.
* etc/refcards/cs-dired-ref.tex, etc/refcards/cs-refcard.tex:
* etc/refcards/cs-survival.tex, etc/refcards/sk-dired-ref.tex:
* etc/refcards/sk-refcard.tex, etc/refcards/sk-survival.tex:
Convert from iso-latin-2 to utf-8.  Remove no-longer-necessary
coding comments and "\input utf8off" directives.