Remove two unused dev dependencies

package-lock.json

@@ -5987,15 +5987,6 @@
         "pinkie": "^2.0.0"
       }
     },
-    "pixelmatch": {
-      "version": "4.0.2",
-      "resolved": "https://registry.npmjs.org/pixelmatch/-/pixelmatch-4.0.2.tgz",
-      "integrity": "sha1-j0fc7FARtHe2fbA8JDvB8wheiFQ=",
-      "dev": true,
-      "requires": {
-        "pngjs": "^3.0.0"
-      }
-    },
     "plugin-error": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/plugin-error/-/plugin-error-1.0.1.tgz",
@@ -6013,12 +6004,6 @@
       "resolved": "https://registry.npmjs.org/pn/-/pn-1.1.0.tgz",
       "integrity": "sha512-2qHaIQr2VLRFoxe2nASzsV6ef4yOOH+Fi9FBOVH6cqeSgUnoyySPZkxzLuzd+RYOQTRpROA0ztTMqxROKSb/nA=="
     },
-    "pngjs": {
-      "version": "3.4.0",
-      "resolved": "https://registry.npmjs.org/pngjs/-/pngjs-3.4.0.tgz",
-      "integrity": "sha512-NCrCHhWmnQklfH4MtJMRjZ2a8c80qXeMlQMv2uVp9ISJMTt562SbGd6n2oq0PaPgKm7Z6pL9E2UlLIhC+SHL3w==",
-      "dev": true
-    },
     "posix-character-classes": {
       "version": "0.1.1",
       "resolved": "https://registry.npmjs.org/posix-character-classes/-/posix-character-classes-0.1.1.tgz",

package.json

@@ -72,8 +72,6 @@
     "js-green-licenses": "^2.0.1",
     "jshint": "^2.11.0",
     "mocha": "^6.2.3",
-    "pixelmatch": "^4.0.2",
-    "pngjs": "^3.4.0",
     "rimraf": "^2.6.3",
     "selenium-standalone": "^6.17.0",
     "through2": "^3.0.1",