Remove old storybook configuration (#545)

In https://github.com/spotify/backstage/pull/371, storybook config was moved from packages/core to packages/story to avoid conflicts.
packages/core/.storybook was missed and not removed at the time. Now, we don't need this anymore.

packages/core/.storybook/main.js

@@ -1,22 +0,0 @@
-module.exports = {
-  stories: [
-    '../src/layout/**/*.stories.tsx',
-    '../src/components/**/*.stories.tsx',
-  ],
-  addons: ['@storybook/addon-actions', '@storybook/addon-links'],
-  webpackFinal: async config => {
-    config.module.rules.push({
-      test: /\.(ts|tsx)$/,
-      use: [
-        {
-          loader: require.resolve('ts-loader'),
-          options: {
-            transpileOnly: true,
-          },
-        },
-      ],
-    });
-    config.resolve.extensions.push('.ts', '.tsx');
-    return config;
-  },
-};