Remove gulp-imagemin

This package hasn't been used since #8140, which dropped it for being
too slow and of minimal benefit.

We should consider re-adding this as a CI check to ensure images are
optimized, but I don't think it should be re-added to the build process
itself.

package.json

@@ -243,7 +243,6 @@
     "gulp-autoprefixer": "^5.0.0",
     "gulp-babel": "^8.0.0",
     "gulp-debug": "^3.2.0",
-    "gulp-imagemin": "^6.1.0",
     "gulp-livereload": "4.0.0",
     "gulp-multi-process": "^1.3.1",
     "gulp-rename": "^2.0.0",
@@ -302,9 +301,6 @@
   "lavamoat": {
     "allowScripts": {
       "node-sass": true,
-      "optipng-bin": true,
-      "gifsicle": true,
-      "jpegtran-bin": true,
       "chromedriver": true,
       "geckodriver": true,
       "@sentry/cli": true,