[REF] web: Remove is_admin key in session info

Because of its redundency with is_system key.

addons/web/models/ir_http.py

@@ -26,7 +26,6 @@ class Http(models.AbstractModel):
             "session_id": request.session.sid,
             "uid": request.session.uid,
             "is_system": request.env.user._is_system(),
-            "is_admin": request.env.user.has_group('base.group_system'),
             "is_superuser": request.env.user._is_superuser(),
             "user_context": request.session.get_context() if request.session.uid else {},
             "db": request.session.db,

addons/web/static/src/js/widgets/debug_manager.js

@@ -64,7 +64,7 @@ var DebugManager = Widget.extend({
         // whether group is currently enabled for current user
         this._has_features = false;
         // whether the current user is an administrator
-        this._is_admin = session.is_admin;
+        this._is_admin = session.is_system;
         return $.when(
             new Model('res.users').call('check_access_rights', {operation: 'write', raise_exception: false}),
             session.user_has_group('base.group_no_one'),