vcc: Remove stale assertion

Fixes #3960

bin/varnishtest/tests/r03960.vtc

+varnishtest "invalid header name on RHS"
+
+varnish v1 -errvcl "Expected ID got '0'" {
+	vcl 4.1;
+	backend default none;
+	sub vcl_recv {
+		if (req.http.0) {}
+	}
+}

lib/libvcc/vcc_expr.c

@@ -370,7 +370,6 @@ vcc_Eval_Var(struct vcc *tl, struct expr **e, struct token *t,
 {
 
 	(void)type;
-	assert(sym->kind == SYM_VAR);
 	vcc_AddUses(tl, t, NULL, sym, XREF_READ);
 	ERRCHK(tl);
 	*e = vcc_mk_expr(sym->type, "%s", sym->rname);