Remove Banner mechanism.

This patch removes the Banner mechanism, with the last remaining
Banner file replaced by a contrib.texi entry.  This accords with the
principle that the output of running libc.so.6 is not the place to
credit particular contributions (the manual is), and with all other
configuration options not mentioned there, it doesn't seem appropriate
to focus there on the one question of whether the one remaining piece
configured as an add-on was enabled or not.

Tested for x86_64.

	* csu/Makefile (generated): Do not add version-info.h.
	(before-compile): Likewise.
	(all-Banner-files): Remove variable.
	($(objpfx)version-info.h): Remove rule.
	* csu/version.c (banner): Do not include "version-info.h".
	* libidn/Banner: Remove.
	* manual/contrib.texi (Simon Josefsson): New entry.