Remove CodeMirror dependencies (#18911)

EasyMDE already loads all the modes and plugins it needs, so there is no reason for neither the webpack dependency nor the vendored copy

package.json

@@ -10,7 +10,6 @@
     "@claviska/jquery-minicolors": "2.3.6",
     "@primer/octicons": "16.3.1",
     "add-asset-webpack-plugin": "2.0.1",
-    "codemirror": "5.65.2",
     "css-loader": "6.6.0",
     "dropzone": "6.0.0-beta.2",
     "easymde": "2.16.1",