Remove the remaining PRODUCTION build-target usage

After the previous patch we now have only *a single* `PRODUCTION` occurrence in the entire code-base, more specifically in the `web/viewer.html` file.
This special build-target can be replaced with any condition that always evaluate to `false`, such as e.g. a comment.

*Please note:* This patch might be considered too hacky, hence I completely understand if it's rejected.

gulpfile.js

@@ -96,7 +96,6 @@ const AUTOPREFIXER_CONFIG = {
 const BABEL_TARGETS = ENV_TARGETS.join(", ");
 
 const DEFINES = Object.freeze({
-  PRODUCTION: true,
   SKIP_BABEL: true,
   TESTING: undefined,
   // The main build targets:

web/viewer.html

@@ -49,7 +49,7 @@ See https://github.com/adobe-type-tools/cmap-resources
 <!--<script src="resource://pdf.js/web/viewer.js"></script>-->
 <!--#elif !MINIFIED-->
 <!--<script src="viewer.js"></script>-->
-<!--#elif !PRODUCTION-->
+<!--#elif /* Development mode. */-->
     <link rel="resource" type="application/l10n" href="locale/locale.properties">
 
     <script defer src="../node_modules/es-module-shims/dist/es-module-shims.js"></script>