Remove code unused in next release

Since https://github.com/uBlockOrigin/uBlock-issues/issues/156
won't be fixed in next release, no need to ship
with code which will be unused, and anyways only once
the fix is worked on will it be clear exactly what needs
to be used by scriptlets to deal harmoniously with
property listener collisions.

assets/resources/scriptlets.js

@@ -27,8 +27,6 @@
 (function() {
 // >>>> start of private namespace
 'use strict';
-// This a private safe available for use by all injected scriptlets.
-const uBOSafe = new Map();  // jshint ignore: line
 
 
 

src/js/scriptlet-filtering.js

@@ -396,7 +396,6 @@
         out.unshift(
             '(function() {',
             '// >>>> start of private namespace',
-            'const uBOSafe = new Map();',
             '',
         );
         out.push(