[optimizer] remove support for .jsx extension (#12712)

docs/development/core/development-modules.asciidoc

@@ -62,17 +62,14 @@ Here is how import/require statements are resolved to a file:
   * the first of the following paths that resolves to a **file** is our match
     ** path + '.js'
     ** path + '.json'
-    ** path + '.jsx'
     ** path + '.less'
     ** path
     ** path/${basename(path)} + '.js'
     ** path/${basename(path)} + '.json'
-    ** path/${basename(path)} + '.jsx'
     ** path/${basename(path)} + '.less'
     ** path/${basename(path)}
     ** path/index + '.js'
     ** path/index + '.json'
-    ** path/index + '.jsx'
     ** path/index + '.less'
     ** path/index
-  * if none of the above paths matches then an error is thrown
+  * if none of the above paths matches then an error is thrown

src/core_plugins/tests_bundle/index.js

@@ -50,7 +50,7 @@ export default (kibana) => {
 
         if (config.get('tests_bundle.instrument')) {
           env.addPostLoader({
-            test: /\.jsx?$/,
+            test: /\.js$/,
             exclude: /[\/\\](__tests__|node_modules|bower_components|webpackShims)[\/\\]/,
             loader: 'istanbul-instrumenter'
           });

src/optimize/base_optimizer.js

@@ -134,7 +134,7 @@ export default class BaseOptimizer {
           { test: /\.(woff|woff2|ttf|eot|svg|ico)(\?|$)/, loader: 'file-loader' },
           { test: /[\/\\]src[\/\\](core_plugins|ui)[\/\\].+\.js$/, loader: loaderWithSourceMaps('rjs-repack-loader') },
           {
-            test: /\.jsx?$/,
+            test: /\.js$/,
             exclude: babelExclude.concat(this.env.noParse),
             loader: 'babel-loader',
             query: babelOptions.webpack
@@ -145,7 +145,7 @@ export default class BaseOptimizer {
       },
 
       resolve: {
-        extensions: ['.js', '.json', '.jsx', '.less', ''],
+        extensions: ['.js', '.json', '.less', ''],
         postfixes: [''],
         modulesDirectories: ['webpackShims', 'node_modules'],
         fallback: [fromRoot('webpackShims'), fromRoot('node_modules')],