Ignore nested node_modules and delete postcss (#1828)

* Ignore nested node_modules and delete postcss

Add an explicit '!**/node_modules/**' ignore pattern in Gruntfile.js so nested node_modules folders are excluded from file sets. Remove modules/user-directory/postcss.config.js (Tailwind + Autoprefixer) as the per-module PostCSS config is no longer required.

* Remove redundant node_modules ignore pattern

Simplify Gruntfile.js main.src globs by removing the duplicate '!node_modules/**' entry. The existing '!**/node_modules/**' already excludes node_modules at any depth (including top-level), so this cleans up the pattern list without changing behavior.

Author: arifulhoque7

Gruntfile.js

@@ -190,7 +190,7 @@ module.exports = function( grunt) {
             main: {
                 src: [
                     '**',
-                    '!node_modules/**',
+                    '!**/node_modules/**',
                     '!build/**',
                     '!admin/form-builder/assets/**',
                     '!assets/css/*.less',