Remove unused `gulp-{if, postcss, sourcemaps}` dependencies

package.json

@@ -60,11 +60,8 @@
     "focus-visible": "^5.0.0",
     "gulp": "^4.0.0",
     "gulp-changed": "^4.0.0",
-    "gulp-if": "^3.0.0",
-    "gulp-postcss": "^8.0.0",
     "gulp-rename": "^1.2.2",
     "gulp-replace": "^1.0.0",
-    "gulp-sourcemaps": "^2.6.4",
     "gulp-util": "^3.0.7",
     "hammerjs": "^2.0.4",
     "jquery": "^3.2.1",