Remove dead ports

Remove the following ports:

- dgux
- nextstep
- sunos4
- svr4
- ultrix4
- univel

These are obsolete and not worth rescuing.  In most cases, there is
circumstantial evidence that they wouldn't work anymore anyway.

src/backend/main/main.c

@@ -224,10 +224,6 @@ startup_hacks(const char *progname)
 	 */
 #ifdef NOFIXADE
 
-#if defined(ultrix4)
-	syscall(SYS_sysmips, MIPS_FIXADE, 0, NULL, NULL, NULL);
-#endif
-
 #if defined(__alpha)			/* no __alpha__ ? */
 	{
 		int			buffer[] = {SSIN_UACPROC, UAC_SIGBUS | UAC_NOPRINT};