Play Framework

chore: remove Selenium support from testrunner + framework

Selenium has been deprecated upstream for years; the legacy in-browser test
harness (TestRunner.html, RemoteRunner, scriptaculous, prototype.js, the
htmlutils/cssQuery shims, and the bundled javascript-xpath) is replaced by
JUnit-based unit and functional tests that run via FirePhoque/HtmlUnit.

- Drops the selenium FastTag (framework/templates/tags/selenium.html),
  TestEngine.allSeleniumTests + scanForSeleniumTests, and the matching
  S: lane in FirePhoque + TestRunner.list.
- Removes modules/testrunner/public/test-runner/selenium/ entirely
  (Selenium IDE assets, scriptaculous, cssQuery, sizzle, snapsie, the
  Selenium-side TestRunner.html shell and its icons).
- Removes the .test.html template path in TestRunner controller and
  the matching index.html JS that bootstrapped the Selenium iframe.
- Drops the obsolete application-skel/test/Application.test.html stub
  and the docs (cheatsheet ch23, manual/test.textile Selenium section).
- LegacyMockMailSystem comment cleanup (the static map was kept "for
  Selenium tests" — that justification no longer applies, but the field
  stays for serial functional-test access).

framework/src/play/test/Helpers.java is also dropped — it only existed to
proxy Selenium calls.