Remove support for vax-openbsd

This removes the support for vax-openbsd which has been discontinued
after the OpenBSD 5.9 release and which has no supported gas or GNU ld
configuration [anymore].  In particular this target does only support
STABS debuginfo generation.

2021-09-13  Richard Biener  <rguenther@suse.de>

	* config.gcc: Remove vax-*-openbsd* configuration.

contrib/
	* config-list.mk: Remove vax-openbsd.