selenium: stop running extensions/skins specs

selenium: stop running extensions/skins specs

So that we can use different framework and let developers easily upgrade
webdriver.io, selenium tests for extensions and skins must now be run
directly at the root of the extension or skin:

  cd extensions/Foobar
  npm install
  npm run-script selenium-test

All repositories had the entry point introduced and Quibble 0.0.34
supports that new scheme. Thus Wikimedia CI runs extensions / skins
tests twice: once via mediawiki/core and once via the 'selenium-test'
script.

Remove the glob specs for extensions / skins from wdio.conf.

Bug: T199116
Change-Id: If19db4e45b56989e2c15240a9ed0e8a55a940fb9