Bug 1770896 - Remove unused extensions.dtd file, r=mixedpuppy

Differential Revision: https://phabricator.services.mozilla.com/D147151

toolkit/locales/en-US/chrome/mozapps/extensions/extensions.dtd

@@ -1,6 +0,0 @@
-<!-- This Source Code Form is subject to the terms of the Mozilla Public
-   - License, v. 2.0. If a copy of the MPL was not distributed with this
-   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
-
-<!-- addon actions -->
-<!ENTITY setting.learnmore "Learn More…">