export back-ends: Remove duplicate information
* lisp/ox-ascii.el: Remove comments at the beginning of the file since
the library is documented in Org manual.
* lisp/ox-beamer.el: Remove comments at the beginning of the file since
the library is documented in Org manual.
* lisp/ox-html.el: Remove comments at the beginning of the file since
the library is documented in Org manual.
* lisp/ox-icalendar.el: Remove comments at the beginning of the file since
the library is documented in Org manual.
* lisp/ox-md.el: Remove comments at the beginning of the file since
the library is documented in Org manual.