Rollback addition of va_start.

va_start was added in 0f754e29dd, however
it has the wrong number of args and it's not usable in non-variadic
functions anyway so it breaks things (for example Solaris 2.6 as
reported by Tom G. Christensen).i  ok djm@

openbsd-compat/bsd-asprintf.c

@@ -47,7 +47,6 @@ vasprintf(char **str, const char *fmt, va_list ap)
 	char *string, *newstr;
 	size_t len;
 
-	va_start(ap);
 	VA_COPY(ap2, ap);
 	if ((string = malloc(INIT_SZ)) == NULL)
 		goto fail;