Remove support for SingleEntryPlugin

Author: Morten N.O. Henriksen

packages/react-dev-utils/ModuleNotFoundPlugin.js

@@ -100,14 +100,7 @@ class ModuleNotFoundPlugin {
     const { prettierError } = this;
     compiler.hooks.make.intercept({
       register(tap) {
-        // "SingleEntryPlugin" can be removed when Webpack 4 no longer have to be supported
-        if (
-          !(
-            tap.name === 'MultiEntryPlugin' ||
-            tap.name === 'SingleEntryPlugin' ||
-            tap.name === 'EntryPlugin'
-          )
-        ) {
+        if (!(tap.name === 'MultiEntryPlugin' || tap.name === 'EntryPlugin')) {
           return tap;
         }
         return Object.assign({}, tap, {