Remove leftover POS docs example files after new docs rollout (#4518)

These files under docs/surfaces/point-of-sale/staticPages/examples/ and
docs/surfaces/point-of-sale/mdxExamples/ were missed in the cleanup after
the new docs rollout. Their examples now live in the docs repo as code
block source files, so they're no longer needed here.

Also removes the now-stale 'mdxExamples/' eslint ignore pattern.

Author: henryStelle

.eslintrc.js

@@ -7,7 +7,6 @@ module.exports = {
   ignorePatterns: [
     'build/',
     'examples/',
-    'mdxExamples/',
     'node_modules/',
     'packages/*/build/',
     'packages/*/*.d.ts',