Revert "Formally expose VSB_tofile in libvarnishapi"

This reverts commit 79536662.

It shouldn't be exposed in libvarnishapi the way it is in this change
because it creates two diverging symbol versions (for systems with
linker support for symbol versioning) without changing the soname,
which of course we want to avoid in the first place in a stable branch.

If the requirement comes up we can revisit this topic and ensure both
binary stability and compatility (upgrade path).

Refs #3268

lib/libvarnishapi/libvarnishapi.map

 /*-
- * Copyright (c) 2011-2020 Varnish Software AS
+ * Copyright (c) 2011-2017 Varnish Software AS
  * All rights reserved.
  *
  * Author: Tollef Fog Heen <tfheen@varnish-software.com>
@@ -140,11 +140,3 @@ LIBVARNISHAPI_2.0 {
     local:
 	*;
 };
-
-LIBVARNISHAPI_2.1 {
-    global:
-	# vsb.c
-	VSB_tofile;
-    local:
-	*;
-};
\ No newline at end of file