Remove the "div-css" gulp task (PR 15968 follow-up)

After the compatibility updates in PR 15968 it's no longer strictly necessary to build the `viewer.css` file in order for the *development viewer* to work in Chromium-based browsers.

*Please note:* Given that Chromium-based browsers still don't support the *unprefixed* `mask-image` property the icons won't look right, however the development viewer itself works.
Given that Firefox is the *primary* development target, and that running `gulp generic` locally will generate polyfilled CSS, it seems reasonable to make this simplification here.

gulpfile.js

@@ -2004,27 +2004,6 @@ gulp.task(
   )
 );
 
-gulp.task("dev-css", function createDevCSS() {
-  console.log();
-  console.log("### Building development CSS");
-
-  const defines = builder.merge(DEFINES, { GENERIC: true, TESTING: true });
-  const cssDir = BUILD_DIR + "dev-css/";
-
-  return merge([
-    gulp.src("web/images/*", { base: "web/" }).pipe(gulp.dest(cssDir)),
-
-    preprocessCSS("web/viewer.css", defines)
-      .pipe(
-        postcss([
-          postcssDirPseudoClass(),
-          autoprefixer({ overrideBrowserslist: ["last 1 versions"] }),
-        ])
-      )
-      .pipe(gulp.dest(cssDir)),
-  ]);
-});
-
 gulp.task(
   "dev-sandbox",
   gulp.series(
@@ -2053,13 +2032,6 @@ gulp.task(
 gulp.task(
   "server",
   gulp.parallel(
-    function watchDevCSS() {
-      gulp.watch(
-        ["web/*.css", "web/images/*"],
-        { ignoreInitial: false },
-        gulp.series("dev-css")
-      );
-    },
     function watchDevFitCurve() {
       gulp.watch(
         ["src/display/editor/*"],

web/viewer-geckoview.js

@@ -61,13 +61,6 @@ function getViewerConfiguration() {
 function webViewerLoad() {
   const config = getViewerConfiguration();
   if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")) {
-    if (window.chrome) {
-      const link = document.createElement("link");
-      link.rel = "stylesheet";
-      link.href = "../build/dev-css/viewer.css";
-
-      document.head.append(link);
-    }
     window.isGECKOVIEW = true;
 
     import("pdfjs-web/genericcom.js").then(function (genericCom) {

web/viewer.js

@@ -207,14 +207,6 @@ function getViewerConfiguration() {
 function webViewerLoad() {
   const config = getViewerConfiguration();
   if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")) {
-    if (window.chrome) {
-      const link = document.createElement("link");
-      link.rel = "stylesheet";
-      link.href = "../build/dev-css/viewer.css";
-
-      document.head.append(link);
-    }
-
     Promise.all([
       import("pdfjs-web/genericcom.js"),
       import("pdfjs-web/pdf_print_service.js"),