Make the fix for using DDT with jQuery and RequireJS more focused

When we undefine the entire window.define, modules loaded asynchronously
can randomly fail to register themselves. However, jQuery 2+ checks for
window.define.amd before registering itself as an AMD module, not just
window.define. So, by unsetting window.define.amd instead of
window.define, we don't break define() for other modules and we still
prevent the page-global jQuery from registering itself as an AMD module,
which was the ultimate goal of #591.

debug_toolbar/static/debug_toolbar/js/jquery_post.js

@@ -1 +1,4 @@
-var djdt = {jQuery: jQuery.noConflict(true)}; window.define = _djdt_define_backup;
+var djdt = {jQuery: jQuery.noConflict(true)};
+if (typeof(window.define) !== 'undefined') {
+    window.define.amd = _djdt_define_amd_backup;
+}

debug_toolbar/static/debug_toolbar/js/jquery_pre.js

@@ -1 +1,5 @@
-var _djdt_define_backup = window.define; window.define = undefined;
+var _djdt_define_amd_backup;
+if (typeof(window.define) !== 'undefined') {
+    _djdt_define_amd_backup = window.define.amd;
+    window.define.amd = undefined;
+}